Rapidghoul
2011-11-25, 07:13 PM
For a while now I've been playing around with homebrew for my final fantasy crystal chronicles campaign. I already have races (http://www.giantitp.com/forums/showthread.php?t=180576) for the world, but I've also worked a lot at classes and prestige classes. Before I implement one of them into the game (as the antagonist of the main story) I want to make sure it's properly balanced and fluffed.
The class in question is the Dark Knight (http://finalfantasy.wikia.com/wiki/Dark_Knight). The class is meant to feel like a spell-less paladin of tyranny, but it borrows from other classes (warlock, favored soul, DFA, hexblade, and samurai) and the fiendish / half-fiendish templates.
Over the updates it's received, Dark Knight has gotten a lot of buffing (as all the early comments said it was far too weak). It now stands as a fiend-themed, fear-focused debuffer melee combatant.
http://images.wikia.com/finalfantasy/images/f/f0/Cecil_D_alt3.png
The Dark Knight
{table=head]Level|BAB|Fort|Ref|Will|Special
1st|+1|+2|+0|+0|Aura of evil, souleater 1d10, terror strike 1/day
2nd|+2|+3|+0|+0|Frightful gaze, infernal toughness
3rd|+3|+3|+1|+1|Aura of fear 5ft, mettle, souleater 2d10
4th|+4|+4|+1|+1|Darkvision 30ft, terror strike 2/day
5th|+5|+4|+1|+1|Fiendish presence, souleater 3d10
6th|+6/+1|+5|+2|+2|Aura of fear 10ft, improved gaze
7th|+7/+2|+5|+2|+2|Greater terror strike, souleater 4d10
8th|+8/+3|+6|+2|+2|Terror strike 3/day, horrific wave
9th|+9/+4|+6|+3|+3|Aura of fear 15ft, souleater 5d10
10th|+10/+5|+7|+3|+3|Dark wave, fiendish resistance
11th|+11/+6/+1|+7|+3|+3|Darkvision 60ft, souleater 6d10
12th|+12/+7+2|+8|+4|+4|Aura of fear 20ft, terror strike 4/day
13th|+13/+8/+3|+8|+4|+4|Souleater 7d10
14th|+14/+9/+4|+9|+4|+4|Greater gaze, debilitating wave
15th|+15/+10/+5|+9|+5|+5|Aura of fear 25ft, fiendish resolve, souleater 8d10
16th|+16/+11/+6/+1|+10|+5|+5|Terror strike 5/day
17th|+17/+12/+7/+2|+10|+5|+5|Souleater 9d10
18th|+18/+13/+8/+3|+11|+6|+6|Aura of fear 30ft, dire terror strike
19th|+19/+14/+9/+4|+11|+6|+6|Souleater 10d10, true darkvision
20th|+20/+15/+10/+5|+12|+6|+6|Devastating wave, fiendish champion, terror strike 6/day
[/table]
Alignment: Lawful Evil.
Hit Die: d8.
Class Skills: Concentration, Craft, Handle Animal, Intimidate, Knowledge(nobility and royalty), Knowledge (religion), Profession, Ride, Sense Motive, and Survival.
Skill Points at 1st level: (2 + Int modifier) x 4.
Skill Points at each additional level: 2 + Int modifier.
Dark Knight Features:
Weapon and Armor Proficiency: Dark Knights are proficient with all simple and martial weapons, with all types of armor, and with shields (but not tower shields).
Bonus Languages: A Dark Knight's bonus language options include Infernal.
Aura of Evil (Ex): Your aura of evil is equal to your Dark Knight level, just like the aura of a cleric of an evil deity.
Everything so far is more or less the same as a paladin of tyranny. Skills, proficiencies, aura of evil. Adding Infernal as a bonus language is mostly for fluff.
Terror Strike (Su): Once per day, as a free action, a Dark Knight can unleash fear upon a foe they strike with a melee attack, giving them a -2 penalty on attacks, saves, ability checks and skill checks for 1 hour. A successful Will save (DC 10 + 1/2 your Dark Knight level + their Cha modifier) negates the effect.
At 4th level, and at every four levels thereafter, you may use Terror Strike one additional time per day. Any effect that removes or dispels fear eliminates the effect of your Terror Strike.
Terror Strike is meant to be similar to the Hexblade's Curse ability but thematically suited for a fear-based class instead of a curse based class. This means no damage penalty (so it reflects the Shaken step in fear). It also is an on-hit ability rather than a ranged point-at-it effect since Dark Knight is a melee class without Hexblade's flashy spells, and it gives them more reason to get into melee. This is purposefully NOT a regular fear effect so its penalties stack separately, essentially representing a deeper fear effect without escalating to higher levels.
Souleater (Su): A Dark Knight is able to infuse their weapon with some of their own life-force to emit a deadly wave of energy. Souleater takes the form of a cone that deals 1d10 points of damage to all enemies within a range of 25 feet + 5 feet / 2 levels. A successful Will save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) halves the damage. Creatures immune to death effects (such as constructs and undead) automatically succeed on this save. Whenever you use souleater, you take 1d3 points of damage.
Souleater's area damage increases by 1d10 and the damage you take increases by 1d3 at each odd Dark Knight level. For example, a 15th level Dark Knight using souleater deals 8d10 damage to enemies in the area and takes 8d3 damage in doing so.
The big fluff and flavor of the Dark Knight is Souleater (or Dark Wave depending on which translation of final fantasy you play). The idea is damaging everything on the field while taking damage yourself.
The ability is like Dragonfire Adept's breath weapon or Warlock's blast. Damage is increased from a d6 progression to d10 since you're also taking a little damage and miss out on some of the flexibility of the other classes' abilities (as seen below with the Wave abilities). Since the class is meant to be tanking and melee based, the wave is not meant to be nearly as good as the others, but letting it deal more damage fits the theme fine.
Frightful Gaze (Ex): At 2nd level, you gain a +2 bonus on Intimidate checks and can demoralize an opponent within 30 feet as a standard action.
Essentially stolen from Samurai, this is meant to help make using Intimidate and stacking fear effects a better option for the Dark Knight. It also provides a ranged option for the class that doesn't cost hit points.
Infernal Toughness (Su): Beginning at 2nd level, you gain bonus hit points equal to your Cha modifier x your Dark Knight level.
Pure and simple, this is a buffer for Souleater's hit point cost. If you never use Souleater, you have nice extra hit points, but what's the fun in that? It also encourages focusing on Cha even more, reducing the need for Con and giving the class a little less MAD. The trade-off is reducing the hit die from d10 (as a paladin) to d8 (as an outsider) so the hit points aren't quite as overwhelming and to keep with the theme even more.
Aura of Fear (Su): Beginning at 3rd level, you area immune to fear (magical or otherwise). Each enemy within 5 feet of you must succeed on a Will save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) or become shaken. A creature who successfully saves cannot be affected by your fear aura for 24 hours.
At 6th level and every three levels thereafter, your aura of fear's radius increases by 5 feet to a max of 30 feet at 18th level.
This ability functions while you are conscious, but not while unconscious or dead.
A scaling effect like that of the same name from Dread Necromancer, but still kept in short range to encourage getting in the middle of a fight. Any fear-based class deserves fear immunity, especially a paladin-themed one.
This is still only the second fear effect the class receives, but combining a move action to get into range and a standard action to demoralize your target becomes a strong alternative to swinging your sword or using Souleater.
Mettle (Ex): At 3rd level and higher, you can resist magical and unusual attacks with great willpower or fortitude. If you make a successful Will or Fortitude save against an attack that normally would have a lesser effect on a successful save, you instead completely negate the effect. You do not gain this benefit while unconscious or sleeping.
Rather than gaining immunity to poisons or disease, giving Mettle as a Hexblade gets made more sense. More than anything else, this is just a strong boost to the class to give it an edge against casters.
Darkvision (Ex): At 4th level, you gain darkvision out to 30 feet. At 11th level, this increases to 60 feet.
Simply because baatezu have darkvision. This also progresses up to See in Darkness at 19th level.
Fiendish Presence (Su): At 5th level, you begin to tap into the dark powers of Hell, gaining resistance to fire 5, +1 bonus to natural armor, and damage reduction 2/magic. Any melee weapon you wield is treated as a magic weapon for the purpose of overcoming damage reduction. For purposes of Intimidate checks, you are treated as one size-category larger than you actually are.
Over the course of four abilities, the class is meant to give the character the aspects of a devil. Looking at the fiendish and half-fiend templates, this means things like damage reduction, natural armor, acid, cold, and fire resist, etc. I didn't want to give the SLA from the templates since this is meant to be a spell-less class.
The DR and Natural Armor scales on a similar progression as various classes (Warlock, Dragonfire Adept, etc.). The overcoming-DR part is an ACF of paladin and fluffy. The size boost to Intimidate is part flavor, part making this level interesting; it's not meant to be extremely powerful since the templates' bonuses increase with HD, but looking more devilish before actually becoming so should still grant some benefit. Fire resist is due to baatezu getting fire immunity.
Improved Gaze (Ex): At 6th level, you gain an additional +2 bonus on Intimidate checks and can demoralize a number of opponents up to your Cha modifier within 30 feet.
Again, stolen from Samurai, but with another Intimidate bonus to solidify the option as well as make up for the low skill points and MAD; a Dark Knight doesn't need to be smart, so likely Int will a big dump stat. All the Intimidate bonuses allows a character to put their points in some other option if they really want or to optimize further if they really want.
Greater Terror Strike (Su): At 7th level, the penalty on attacks, saves, ability checks, and skill checks incurred by your terror strike becomes -4 instead of -2.
As with Hexblade's Curse, Terror Strike's penalties increase with level. This way the debuff options don't become obsolete at later levels. Nothing more to say.
Horrific Wave (Su): At 8th level, your souleater strikes fear into the hearts of your enemies by taking the form of their worst nightmare. As a swift action, you can enhance your souleater to cause any creature it damages to become frightened for 1 round. On a successful Will save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) the creature is instead shaken for 1 round.
The first debuffing-wave effect, similar to a Warlock's Essences or a Dragonfire Adept's Breath Effects. Wave effects are meant to be weaker debuffs since the damage is already boosted compared to those classes, but they still have a decent effect in combat. I kept the enhancement as a swift action since the class doesn't have any swift action abilities yet, and the effects have no real downside to the wave; any time you use Souleater, you'll be added a wave effect to it, so lowering the time down to a free action seems unnecessary.
I debate on upping frightened to panicked since it's only for 1 round, but with all the options the class has for scaring the enemy, frightened should be fine, especially on an area effect.
Dark Wave (Su): At 10th level, your souleater becomes virulent enough to able to obstruct view. As a swift action, you can enhance your souleater to cause any enemy it damages to become blind for 1 round. On a successful Fortitude save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) the creature is instead dazzled for 1 round.
The second debuff-wave is meant to give you a non-fear option in combat. Whether it's because fear isn't going to work or because everything is feared already, a different combat-focused debuff is nice. Dark Wave is also an alternate name to Souleater in many of the games, so a class ability of the same name is thematic if nothing else.
Fiendish Resistance (Su): At 10th level, your connection to the fiends strengthens. Your natural armor bonus increases to +2, and your damage reduction improves to DR 5/magic. You also gain resistance to acid and cold 5 and a profane bonus to all saves equal to half your Cha modifier.
Step two in becoming a fiend buffs step one's armor and damage reduction and grants two energy resistances. The bonus to saves is to help make up for MAD without being quite as powerful as Divine Grace.
Exhausting Wave (Su): At 14th level, your souleater becomes more potent, weakening your foes' defenses. As a swift action, you can enhance your souleater to cause any creature that it damages to become exhausted for 1 round. On a successful Fortitude save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) the creature instead becomes fatigued for 1 round.
I chose for the third debuff-wave to be exhaust/fatigue since they target combat-heavy abilities (Str and Dex) which the Dark Knight would obviously care a lot about. It's also a great debuff for those pesky paladins that are immune to fear.
Greater Gaze (Ex): At 14th level, you gain an additional +2 bonus on Intimidate checks and can demoralize opponents within 30 feet as a move action, not a standard action.
Again, a Samurai ability plus an Intimidate bonus. Now that demoralizing an opponent is a move action, you can pull off three fear levels in a single round (assuming they're in range).'
Fiendish Resolve (Su): At 15th level, you draw upon dark powers to make yourself more resilient against the forces of good. Your resistance to fire increases to 10, your natural armor bonus increases to +3, your damage reduction improves to DR 5/good, and any melee weapon you wield is treated as evil-aligned for the purpose of overcoming damage reduction. You also gain spell resistance equal to 5 + your Dark Knight level.
Step three in becoming a devil combines parts of the first two steps. Better fire resist, better armor, better damage reduction. Spell resistance since all baatezu in the monster manual except imps and lemures have SR.
Dire Terror Strike (Su): At 18th level, the penalty on attacks, saves, ability checks, and skill checks incurred by your terror strike becomes -6 instead of -4.
Hexblade's Curse maxes at -6 penalties, as does Terror Strike. 'Nuff said.
True Darkvision (Su): At 19th level, you can see perfectly in darkness of any kind, even that created by a deeper darkness spell.
As per the baatezu subtype's see in darkness. Seeing through even magic darkness is a supernatural ability all devils get.
Devastating Wave (Su): At 20th level, your souleater becomes devastatingly powerful, overwhelming your foes. As a swift action, you can enhance your souleater to cause any creature that it damages to gain 1 negative level. On a successful Fortitude save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) the creature instead takes 1d4 ability damage to the ability of your choice. Multiple uses of Devastating Wave do not stack on the same subject.
The last debuff-wave has always been tricky. It needs to be powerful, but the save-effect and duration become problematic. I played around with paralysis, stuns, slows, fear effects, saves and AC penalties... I finally decided on negative level or ability damage. It's a big effect, but it's a level 20 feature.
Fiendish Champion (Su): At 20th level, you become a champion of the evil powers that empower you. Your type changes to Outsider (Native). You gain resistance to acid and cold 10, immunity to fire and poison, your natural armor bonus increases to +5, your damage reduction improves to DR 10/good, and your profane bonus to saves increases to your full Cha modifier. Finally, you gain batlike wings and can fly at a speed of 60 feet (average maneuverability). If you already have wings, you can choose whether these batlike wings replace your own.
As a capstone of a class that drives you to be a devil, obviously it needs to be kind of big.
Changing types is a capstone feature seen in a lot of classes (Monk, Dread Necromancer, etc.) and is an obvious final step in becoming a devil-like. Not actually gaining the baatezu subtype distinguishes you from real devils, since you can't summon other baatezu and don't get telepathy as they do. Keeping the subtype as Native prevents the troublesome no-resurrection-clause from killing the appeal. You're a devil-on-earth.
Natural armor, acid and cold resist 10, fire and poison immunity, DR 10/good, and SR 25 are all abilities that fiends typically get in one way or another.
Wings are kind of a throwaway benefit at level 20, but they certainly add to the effect.
I've gone back and forth on the idea of giving the class divine grace. On the one hand, it's a pseudo-paladin, so it fits. On the other hand, it has mettle and infernal toughness, both which fit the theme better and make divine grace a little too good. I decided to give the ability as is (half at 10, full at 20) as part of the fiendish progression to make those abilities feel bigger.
Ex-Dark Knights: A Dark Knight who ceases to be lawful evil loses all Dark Knight class features except weapon, armor, or shield proficiencies. They may not progress any farther in levels as a Dark Knight until they atone.
Adaptation: A chaotic Dark Knight ("Demonic Knight" or "Abyssal Knight" or even just "Black Knight") is easily adapted from this class. The fiendish abilities would be modified to mimic the Tanar'ri subtype rather than the Baatezu subtype, such as getting the Abyssal language instead of Infernal, or gaining immunity to electricity instead of fire.
The class in question is the Dark Knight (http://finalfantasy.wikia.com/wiki/Dark_Knight). The class is meant to feel like a spell-less paladin of tyranny, but it borrows from other classes (warlock, favored soul, DFA, hexblade, and samurai) and the fiendish / half-fiendish templates.
Over the updates it's received, Dark Knight has gotten a lot of buffing (as all the early comments said it was far too weak). It now stands as a fiend-themed, fear-focused debuffer melee combatant.
http://images.wikia.com/finalfantasy/images/f/f0/Cecil_D_alt3.png
The Dark Knight
{table=head]Level|BAB|Fort|Ref|Will|Special
1st|+1|+2|+0|+0|Aura of evil, souleater 1d10, terror strike 1/day
2nd|+2|+3|+0|+0|Frightful gaze, infernal toughness
3rd|+3|+3|+1|+1|Aura of fear 5ft, mettle, souleater 2d10
4th|+4|+4|+1|+1|Darkvision 30ft, terror strike 2/day
5th|+5|+4|+1|+1|Fiendish presence, souleater 3d10
6th|+6/+1|+5|+2|+2|Aura of fear 10ft, improved gaze
7th|+7/+2|+5|+2|+2|Greater terror strike, souleater 4d10
8th|+8/+3|+6|+2|+2|Terror strike 3/day, horrific wave
9th|+9/+4|+6|+3|+3|Aura of fear 15ft, souleater 5d10
10th|+10/+5|+7|+3|+3|Dark wave, fiendish resistance
11th|+11/+6/+1|+7|+3|+3|Darkvision 60ft, souleater 6d10
12th|+12/+7+2|+8|+4|+4|Aura of fear 20ft, terror strike 4/day
13th|+13/+8/+3|+8|+4|+4|Souleater 7d10
14th|+14/+9/+4|+9|+4|+4|Greater gaze, debilitating wave
15th|+15/+10/+5|+9|+5|+5|Aura of fear 25ft, fiendish resolve, souleater 8d10
16th|+16/+11/+6/+1|+10|+5|+5|Terror strike 5/day
17th|+17/+12/+7/+2|+10|+5|+5|Souleater 9d10
18th|+18/+13/+8/+3|+11|+6|+6|Aura of fear 30ft, dire terror strike
19th|+19/+14/+9/+4|+11|+6|+6|Souleater 10d10, true darkvision
20th|+20/+15/+10/+5|+12|+6|+6|Devastating wave, fiendish champion, terror strike 6/day
[/table]
Alignment: Lawful Evil.
Hit Die: d8.
Class Skills: Concentration, Craft, Handle Animal, Intimidate, Knowledge(nobility and royalty), Knowledge (religion), Profession, Ride, Sense Motive, and Survival.
Skill Points at 1st level: (2 + Int modifier) x 4.
Skill Points at each additional level: 2 + Int modifier.
Dark Knight Features:
Weapon and Armor Proficiency: Dark Knights are proficient with all simple and martial weapons, with all types of armor, and with shields (but not tower shields).
Bonus Languages: A Dark Knight's bonus language options include Infernal.
Aura of Evil (Ex): Your aura of evil is equal to your Dark Knight level, just like the aura of a cleric of an evil deity.
Everything so far is more or less the same as a paladin of tyranny. Skills, proficiencies, aura of evil. Adding Infernal as a bonus language is mostly for fluff.
Terror Strike (Su): Once per day, as a free action, a Dark Knight can unleash fear upon a foe they strike with a melee attack, giving them a -2 penalty on attacks, saves, ability checks and skill checks for 1 hour. A successful Will save (DC 10 + 1/2 your Dark Knight level + their Cha modifier) negates the effect.
At 4th level, and at every four levels thereafter, you may use Terror Strike one additional time per day. Any effect that removes or dispels fear eliminates the effect of your Terror Strike.
Terror Strike is meant to be similar to the Hexblade's Curse ability but thematically suited for a fear-based class instead of a curse based class. This means no damage penalty (so it reflects the Shaken step in fear). It also is an on-hit ability rather than a ranged point-at-it effect since Dark Knight is a melee class without Hexblade's flashy spells, and it gives them more reason to get into melee. This is purposefully NOT a regular fear effect so its penalties stack separately, essentially representing a deeper fear effect without escalating to higher levels.
Souleater (Su): A Dark Knight is able to infuse their weapon with some of their own life-force to emit a deadly wave of energy. Souleater takes the form of a cone that deals 1d10 points of damage to all enemies within a range of 25 feet + 5 feet / 2 levels. A successful Will save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) halves the damage. Creatures immune to death effects (such as constructs and undead) automatically succeed on this save. Whenever you use souleater, you take 1d3 points of damage.
Souleater's area damage increases by 1d10 and the damage you take increases by 1d3 at each odd Dark Knight level. For example, a 15th level Dark Knight using souleater deals 8d10 damage to enemies in the area and takes 8d3 damage in doing so.
The big fluff and flavor of the Dark Knight is Souleater (or Dark Wave depending on which translation of final fantasy you play). The idea is damaging everything on the field while taking damage yourself.
The ability is like Dragonfire Adept's breath weapon or Warlock's blast. Damage is increased from a d6 progression to d10 since you're also taking a little damage and miss out on some of the flexibility of the other classes' abilities (as seen below with the Wave abilities). Since the class is meant to be tanking and melee based, the wave is not meant to be nearly as good as the others, but letting it deal more damage fits the theme fine.
Frightful Gaze (Ex): At 2nd level, you gain a +2 bonus on Intimidate checks and can demoralize an opponent within 30 feet as a standard action.
Essentially stolen from Samurai, this is meant to help make using Intimidate and stacking fear effects a better option for the Dark Knight. It also provides a ranged option for the class that doesn't cost hit points.
Infernal Toughness (Su): Beginning at 2nd level, you gain bonus hit points equal to your Cha modifier x your Dark Knight level.
Pure and simple, this is a buffer for Souleater's hit point cost. If you never use Souleater, you have nice extra hit points, but what's the fun in that? It also encourages focusing on Cha even more, reducing the need for Con and giving the class a little less MAD. The trade-off is reducing the hit die from d10 (as a paladin) to d8 (as an outsider) so the hit points aren't quite as overwhelming and to keep with the theme even more.
Aura of Fear (Su): Beginning at 3rd level, you area immune to fear (magical or otherwise). Each enemy within 5 feet of you must succeed on a Will save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) or become shaken. A creature who successfully saves cannot be affected by your fear aura for 24 hours.
At 6th level and every three levels thereafter, your aura of fear's radius increases by 5 feet to a max of 30 feet at 18th level.
This ability functions while you are conscious, but not while unconscious or dead.
A scaling effect like that of the same name from Dread Necromancer, but still kept in short range to encourage getting in the middle of a fight. Any fear-based class deserves fear immunity, especially a paladin-themed one.
This is still only the second fear effect the class receives, but combining a move action to get into range and a standard action to demoralize your target becomes a strong alternative to swinging your sword or using Souleater.
Mettle (Ex): At 3rd level and higher, you can resist magical and unusual attacks with great willpower or fortitude. If you make a successful Will or Fortitude save against an attack that normally would have a lesser effect on a successful save, you instead completely negate the effect. You do not gain this benefit while unconscious or sleeping.
Rather than gaining immunity to poisons or disease, giving Mettle as a Hexblade gets made more sense. More than anything else, this is just a strong boost to the class to give it an edge against casters.
Darkvision (Ex): At 4th level, you gain darkvision out to 30 feet. At 11th level, this increases to 60 feet.
Simply because baatezu have darkvision. This also progresses up to See in Darkness at 19th level.
Fiendish Presence (Su): At 5th level, you begin to tap into the dark powers of Hell, gaining resistance to fire 5, +1 bonus to natural armor, and damage reduction 2/magic. Any melee weapon you wield is treated as a magic weapon for the purpose of overcoming damage reduction. For purposes of Intimidate checks, you are treated as one size-category larger than you actually are.
Over the course of four abilities, the class is meant to give the character the aspects of a devil. Looking at the fiendish and half-fiend templates, this means things like damage reduction, natural armor, acid, cold, and fire resist, etc. I didn't want to give the SLA from the templates since this is meant to be a spell-less class.
The DR and Natural Armor scales on a similar progression as various classes (Warlock, Dragonfire Adept, etc.). The overcoming-DR part is an ACF of paladin and fluffy. The size boost to Intimidate is part flavor, part making this level interesting; it's not meant to be extremely powerful since the templates' bonuses increase with HD, but looking more devilish before actually becoming so should still grant some benefit. Fire resist is due to baatezu getting fire immunity.
Improved Gaze (Ex): At 6th level, you gain an additional +2 bonus on Intimidate checks and can demoralize a number of opponents up to your Cha modifier within 30 feet.
Again, stolen from Samurai, but with another Intimidate bonus to solidify the option as well as make up for the low skill points and MAD; a Dark Knight doesn't need to be smart, so likely Int will a big dump stat. All the Intimidate bonuses allows a character to put their points in some other option if they really want or to optimize further if they really want.
Greater Terror Strike (Su): At 7th level, the penalty on attacks, saves, ability checks, and skill checks incurred by your terror strike becomes -4 instead of -2.
As with Hexblade's Curse, Terror Strike's penalties increase with level. This way the debuff options don't become obsolete at later levels. Nothing more to say.
Horrific Wave (Su): At 8th level, your souleater strikes fear into the hearts of your enemies by taking the form of their worst nightmare. As a swift action, you can enhance your souleater to cause any creature it damages to become frightened for 1 round. On a successful Will save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) the creature is instead shaken for 1 round.
The first debuffing-wave effect, similar to a Warlock's Essences or a Dragonfire Adept's Breath Effects. Wave effects are meant to be weaker debuffs since the damage is already boosted compared to those classes, but they still have a decent effect in combat. I kept the enhancement as a swift action since the class doesn't have any swift action abilities yet, and the effects have no real downside to the wave; any time you use Souleater, you'll be added a wave effect to it, so lowering the time down to a free action seems unnecessary.
I debate on upping frightened to panicked since it's only for 1 round, but with all the options the class has for scaring the enemy, frightened should be fine, especially on an area effect.
Dark Wave (Su): At 10th level, your souleater becomes virulent enough to able to obstruct view. As a swift action, you can enhance your souleater to cause any enemy it damages to become blind for 1 round. On a successful Fortitude save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) the creature is instead dazzled for 1 round.
The second debuff-wave is meant to give you a non-fear option in combat. Whether it's because fear isn't going to work or because everything is feared already, a different combat-focused debuff is nice. Dark Wave is also an alternate name to Souleater in many of the games, so a class ability of the same name is thematic if nothing else.
Fiendish Resistance (Su): At 10th level, your connection to the fiends strengthens. Your natural armor bonus increases to +2, and your damage reduction improves to DR 5/magic. You also gain resistance to acid and cold 5 and a profane bonus to all saves equal to half your Cha modifier.
Step two in becoming a fiend buffs step one's armor and damage reduction and grants two energy resistances. The bonus to saves is to help make up for MAD without being quite as powerful as Divine Grace.
Exhausting Wave (Su): At 14th level, your souleater becomes more potent, weakening your foes' defenses. As a swift action, you can enhance your souleater to cause any creature that it damages to become exhausted for 1 round. On a successful Fortitude save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) the creature instead becomes fatigued for 1 round.
I chose for the third debuff-wave to be exhaust/fatigue since they target combat-heavy abilities (Str and Dex) which the Dark Knight would obviously care a lot about. It's also a great debuff for those pesky paladins that are immune to fear.
Greater Gaze (Ex): At 14th level, you gain an additional +2 bonus on Intimidate checks and can demoralize opponents within 30 feet as a move action, not a standard action.
Again, a Samurai ability plus an Intimidate bonus. Now that demoralizing an opponent is a move action, you can pull off three fear levels in a single round (assuming they're in range).'
Fiendish Resolve (Su): At 15th level, you draw upon dark powers to make yourself more resilient against the forces of good. Your resistance to fire increases to 10, your natural armor bonus increases to +3, your damage reduction improves to DR 5/good, and any melee weapon you wield is treated as evil-aligned for the purpose of overcoming damage reduction. You also gain spell resistance equal to 5 + your Dark Knight level.
Step three in becoming a devil combines parts of the first two steps. Better fire resist, better armor, better damage reduction. Spell resistance since all baatezu in the monster manual except imps and lemures have SR.
Dire Terror Strike (Su): At 18th level, the penalty on attacks, saves, ability checks, and skill checks incurred by your terror strike becomes -6 instead of -4.
Hexblade's Curse maxes at -6 penalties, as does Terror Strike. 'Nuff said.
True Darkvision (Su): At 19th level, you can see perfectly in darkness of any kind, even that created by a deeper darkness spell.
As per the baatezu subtype's see in darkness. Seeing through even magic darkness is a supernatural ability all devils get.
Devastating Wave (Su): At 20th level, your souleater becomes devastatingly powerful, overwhelming your foes. As a swift action, you can enhance your souleater to cause any creature that it damages to gain 1 negative level. On a successful Fortitude save (DC 10 + 1/2 your Dark Knight level + your Cha modifier) the creature instead takes 1d4 ability damage to the ability of your choice. Multiple uses of Devastating Wave do not stack on the same subject.
The last debuff-wave has always been tricky. It needs to be powerful, but the save-effect and duration become problematic. I played around with paralysis, stuns, slows, fear effects, saves and AC penalties... I finally decided on negative level or ability damage. It's a big effect, but it's a level 20 feature.
Fiendish Champion (Su): At 20th level, you become a champion of the evil powers that empower you. Your type changes to Outsider (Native). You gain resistance to acid and cold 10, immunity to fire and poison, your natural armor bonus increases to +5, your damage reduction improves to DR 10/good, and your profane bonus to saves increases to your full Cha modifier. Finally, you gain batlike wings and can fly at a speed of 60 feet (average maneuverability). If you already have wings, you can choose whether these batlike wings replace your own.
As a capstone of a class that drives you to be a devil, obviously it needs to be kind of big.
Changing types is a capstone feature seen in a lot of classes (Monk, Dread Necromancer, etc.) and is an obvious final step in becoming a devil-like. Not actually gaining the baatezu subtype distinguishes you from real devils, since you can't summon other baatezu and don't get telepathy as they do. Keeping the subtype as Native prevents the troublesome no-resurrection-clause from killing the appeal. You're a devil-on-earth.
Natural armor, acid and cold resist 10, fire and poison immunity, DR 10/good, and SR 25 are all abilities that fiends typically get in one way or another.
Wings are kind of a throwaway benefit at level 20, but they certainly add to the effect.
I've gone back and forth on the idea of giving the class divine grace. On the one hand, it's a pseudo-paladin, so it fits. On the other hand, it has mettle and infernal toughness, both which fit the theme better and make divine grace a little too good. I decided to give the ability as is (half at 10, full at 20) as part of the fiendish progression to make those abilities feel bigger.
Ex-Dark Knights: A Dark Knight who ceases to be lawful evil loses all Dark Knight class features except weapon, armor, or shield proficiencies. They may not progress any farther in levels as a Dark Knight until they atone.
Adaptation: A chaotic Dark Knight ("Demonic Knight" or "Abyssal Knight" or even just "Black Knight") is easily adapted from this class. The fiendish abilities would be modified to mimic the Tanar'ri subtype rather than the Baatezu subtype, such as getting the Abyssal language instead of Infernal, or gaining immunity to electricity instead of fire.