Help me make this fight epic and challenging

[PhoenixPhyre]

My main group always surprises me. So I have a week to devise a fight that very well might be the final fight of the whole campaign. And it should be suitably epic. Note: the setting is custom, so pretty much anything goes.

The participants (all nominally 18th level, but with high-powered items, so they punch more like 20):
* Great Old One Warlock. Loves telekinesis and has both the pushing and the slowing invocations for EB (plus agonizing, of course).
* AT Rogue. TWF (including feat), with two +2d6 weapons. Both he and the warlock have lucky.
* An Open Hand Monk with a staff of striking. Loves being super mobile (has mobile, so base movement 70 ft/round)
* A Forest Land Druid with a legendary staff that gives healing bonuses and free spells. Mainly plays support but also summons. Has a Fomorian in a jar (don't ask). Loves offensive polymorph.
* A EK fighter NPC. Sub-optimal design, mainly meat-shield. Limited ranged abilities.

The enemies:
* An ancient silver dragon on the verge of becoming a demon prince. He's devoured souls to the point that his physical form is starting to become extra-planar. Very honorable, hates deceit. Lawful in the extreme, to the point of fanaticism. His big thing is judgement--I'd expect a theme of arcane/radiant energies here.
* His "brother", an adult gold dragon. Devoted to the ancient silver. Straight-forward, should have clerical flavor to his brother's arcane/soul-fueled energy.

The terrain:
* This can be anything. It will take place in a pocket realm set in the Abyss, and should be interesting and "fair" to both sides. This is what I need most help with.

The challenge:
* This should be very difficult. It will be the only fight of the day, so I expect them to go all out. Both sides will be fighting smart and the enemies won't retreat. This is more of a formal duel than anything, but it's a duel to the death.

Help me make this one memorable. Please suggest tactics, phases, terrain, stat block pieces, etc.

[Unoriginal]

Hold on, just to make sure I've understood correctly.

A silver dragon who's lawful to the extreme is about to become a chaotic evil demon prince after eating a large quantity of souls?

[Kane0]

- The dragon starts the fight with a level 9 Armor of Agathys effect up and running

- Bone-chilling Aura (replaces Dragon Fear Aura): When a creature starts their turn or moves within within X feet of the dragon on their turn they must succeed on a DC Y Constitution saving throw or gain 1 level of exhaustion. Creatures that a resistant to cold have advantage on the saving throw, creatures that are immune to cold are unaffected.

- Steal Spell (reaction, recharge 5-6): When the dragon sees a beneficial spell (such as Cure Wounds or Haste) cast by a creature within 30' it can choose to gain the benefits of that spell until the end of its effects.

- Adaptive Ward (legendary action, cost 1): The dragon chooses one damage type that has injured it since its last turn. It gains resistance to this damage type until the end of its next turn

And don't forget some awesome icy death throes!

[PhoenixPhyre]

Hold on, just to make sure I've understood correctly.

A silver dragon who's lawful to the extreme is about to become a chaotic evil demon prince after eating a large quantity of souls?

Yes, sort of, minus the "chaotic evil" part. Since it's a custom setting without alignment, "demon prince" is more of a power level and power source statement. A demon prince is a being who has enough power from devouring souls (whether voluntarily bequeathed or stolen doesn't matter) to create and maintain an section (similar to a layer in more standard settings) of the Abyss. They should be the power equivalent of a minor-ish demon prince in more established settings, something on the order of Tiamat in her weakened state. Powerful, but not godlike.

This particular dude has a MISSION and is devoted to that to the fanatical extreme. Most of his souls were voluntarily sacrificed (the dying old people who worshiped him, those dying soldiers who wanted to live on within him), but some were of actual evil people (those that summoned devils to oppress people, murderers, rapists, etc). He's actually a good person--he'll agree to this duel in a pocket dimension because it will protect his people and other innocents from collateral damage so he can fight all out.

He wants to create an actual good/evil split and a real "heaven" and "hell" (a feature this setting intentionally lacks). If he wins, he will actually sacrifice himself (write himself out of existence) to accomplish this (using a plot device). The players are basically the only thing stopping him at this point, mainly because they took one look at his tenets of belief and said "Nope". This is a chaos/law fight, both on the good side of the spectrum.

[PhoenixPhyre]

- The dragon starts the fight with a level 9 Armor of Agathys effect up and running

- Bone-chilling Aura (replaces Dragon Fear Aura): When a creature starts their turn or moves within within X feet of the dragon on their turn they must succeed on a DC Y Constitution saving throw or gain 1 level of exhaustion. Creatures that a resistant to cold have advantage on the saving throw, creatures that are immune to cold are unaffected.

- Steal Spell (reaction, recharge 5-6): When the dragon sees a beneficial spell (such as Cure Wounds or Haste) cast by a creature within 30' it can choose to gain the benefits of that spell until the end of its effects.

- Adaptive Ward (legendary action, cost 1): The dragon chooses one damage type that has injured it since its last turn. It gains resistance to this damage type until the end of its next turn

And don't forget some awesome icy death throes!

Nice! That aura's special because the monk is a silver dragonborn (house-ruled to get +DEX instead of +STR), so she'll be less affected. Both the warlock and the druid like support/buffing spells, and the rogue likes to haste himself, so that will be obnoxious

[Kane0]

Oh, and that slow effect that your warlock gets on his EB? Just ignore it, because Cold don't work on silver dragons

[Ratter]

Ok, step 1... the dragon BECOMES a demon prince! He then sentences them each to a background dependant encounter where things they used to get completely destroyed by are a fair fight because they have their friends with them, after completeing each mini encounter inside this encounter, a mcguffin, (any magic item picked up before this fight) starts speaking to the party, they must weaken the demon prince in order to exterminate him permenantly, or else he'll just reform, have a weaker version of Vecna (or straight Vecna if they can handle it) with thematic/flavour changes, after reducing vecna to 0 HP, while he is reforming, the sentient object should tell them to strike the deforming body in the head, after doing this, they go inside the brain of a chaotic demon prince, have the most ****ed up traps imaginable, every trap should be super trolly and random and STUPID! (I have some ideas for stupid traps if you need some) After getting to the end of this mind puzzle maze thing, they come across a crystal, it is filled with souls, the sentient object alerts you that they cannot be saved, and one of you must become a lich to absorb these souls, as they are about to touch it, the sentient object warns them that the dragon thought up countermeasures, all the coolest and strongest allies/enemies appear to fight your party, after some OHKO's and really cool showed progression, they get to the soul crystal as to prevent the dragon from getting them (someone makes the sacrifice blah blah blah touchy feely moment) they have the philactrophy, and then they break out of the dragons skull, the dragon looks up in terror, as all at once they deliver a coup de'tat

[Kane0]

-Snip-

Eh, I've seen this sort of thing work out well sometimes, but more often falls pretty flat for a couple reasons.
- Individual encounters throw out all teamwork and communication plus dramatically increases the workload for you and wait times for the players.
- Throwing in a mcguffin that the party must rely on feels a lot like deus ex machina, especially if it's the only way to progress the fight
- Getting in the boss' head suddenly turns a combat into a puzzle, which is really jarring for pacing
- Without being managed really carefully you risk some severe 'guess what the DM is thinking'
- So many moving parts and major events certainly feels climactic, but not in the way that the PCs feel awesome for overcoming the challenge. You want them to be victorious based on their actions and decisions against an extremely formidable foe, not because you have a great set of narrated cinematics prepared.

[Hugh Mann]

Tactics:

As a couple of dragons reason suggests that they should use their flight to their advantage, so that they can avoid any melee attacks. All logic says that they should attack using thier breath weapons and only engage in melee when they need to recharge their breath attacks. When in melee they can simply use their superior reach to strike at the PCs while still in the air and use the rest of their movement to fly back up. The dragons should prefer to stay a good distance apart so that they can't get hit by AoEs.
If the party proves resistant to their fire and cold, then they can prioritize paralysis. That way they can do large amounts of damage with crits, as they can also simply use their legendary actions to do more critical attacks.
And if the party also proves to be able to resist paralysis, then the dragons can just grapple one of them (probably the Druid or warlock) and proceed to tear them apart in midair.
Even if the party can fly the dragons still have pretty good fly speed, which might allow them to keep out of range.
And if you really want to be mean, you can have the dragons ready breath attacks to disrupt spell casting.

Although if the Dragons look down upon the PCs then they might engage in melee at first and only do hit and run tactics if things start going south.

Unfortunately, while this might a challenge, it is ultimately kind of boring. But there doesn't seem to be much reason for the dragons to do otherwise. Which is why I think that the terrain should be focused on just as much


Terrain:

Let's add some stakes to this fight. Due to the sheer stress of a nascent demon lord fighting against the PCs the pocket realm begins to destabilize. The influx of planar energies makes it difficult or impossible to plane shift out. The energy causes massive environmental changes that hinder both groups. This being the abyss anything can be possible: the ground breaks apart and boulders start floating upward, massive windstorms, lightning, earthquakes, the size of the pocket realm shrinking, chaos magic effects, null magic, you name it. The longer the fight the worse it gets, and it becomes increasingly apparent that if the fight doesn't end soon the pocket realm will collapse killing everyone inside.

This forces the dragons to switch from a conservative fight style to a more desperate one. Force both groups to take big stupid risks in order to win. Nothing adds to the tension to a situation like a countdown to destruction.
This can also give the party an extra win condition, they can simply draw out the fight effectively sacrificing themselves to trap/kill the dragons in the pocket realm. In this case, the Silver Dragon might be able to find an exit or repair the pocket realm but not so long as the party distracts it.