Vermintide

[ZenBear]

I'm incorporating the Skaven of Warhammer into my custom setting for my campaign. They've already made their first appearance, taking the place of Goblins in the Phandelver adventure. They have gone through a couple revisions since I ran them, and this is what I have so far. At this point only the Slaves, Clanrats and a low level Plague Priest have appeared.

Skaven Slave
AC: 12
HP: 7 (2d6)
Speed: 30 ft.
STR 10 (0) DEX 14 (+2) CON 10 (0)
INT 8 (-1) WIS 12 (+1) CHA 6 (-2)
Sunlight Sensitivity: While in sunlight, the Skaven has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Keen Smell: The Skaven has advantage on Wisdom (Perception) checks that rely on smell.
Cowardly Wretch: The Skaven Slave has disadvantage on saving throws against being frightened unless lead by more a powerful Skaven.
Shortsword: +4, 5(1d6+2)
Sling: +4, 4(1d4+2), 30/120 ft.
Bite: +4, 4(1d4+2)

Clanrat
AC: 14 (hide armor) 16 (shield)
HP: 20 (4d6+4)
Speed: 30 ft.
STR 12 (+1) DEX 14 (+2) CON 12 (+1)
INT 10 (0) WIS 12 (+1) CHA 8 (-1)
Sunlight Sensitivity: While in sunlight, the Skaven has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Keen Smell: The Skaven has advantage on Wisdom (Perception) checks that rely on smell.
Pack Tactics: The Skaven has advantage on an attack roll against a creature if at least one of your allies is within 5 feet of the creature and the ally isn’t incapacitated.
Spear: +4, 5(1d6+2), reach
Spear (versatile): +4, 6(1d8+2), reach
Shortbow: +4, 5(1d6+2), 80/320 ft.
Bite: +4, 4(1d4+2)

Stormvermin
AC: 16 (scale mail) 18 (+shield)
HP: 33 (6d6+12)
Speed: 30ft.
STR 14 (+2) DEX 14 (+2) CON 14 (+2)
INT 10 (0) WIS 12 (+1) CHA 10 (0)
Sunlight Sensitivity: While in sunlight, the Skaven has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Keen Smell: The Skaven has advantage on Wisdom (Perception) checks that rely on smell.
Pack Tactics: The Skaven has advantage on an attack roll against a creature if at least one of your allies is within 5 feet of the creature and the ally isn’t incapacitated.
Martial Advantage: Once per turn, the Stormvermin can deal an extra 7 (2d6) damage to a creature it hits with a weapon attack if that creature is within 5 ft. of an ally of the Stormvermin that isn’t incapacitated.
Brave: The Stormvermin has advantage on saving throws against being frightened.
Multiattack: The Stormvermin makes two attacks with its glaive or longsword.
Glaive: +5, 7(1d10+2), reach
Longsword: +5, 6(1d8+2)
Longbow: +5, 6(1d8+2), 150/600 ft.

Night Runner
AC: 16 (chain shirt)
HP: 27 (6d6+6)
Speed: 30 ft.
STR 12 (+1) DEX 16 (+3) CON 12 (+1)
INT 10 (0) WIS 12 (+1) CHA 8 (-1)
Sunlight Sensitivity: While in sunlight, the Skaven has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Keen Smell: The Skaven has advantage on Wisdom (Perception) checks that rely on smell.
Pack Tactics: The Skaven has advantage on an attack roll against a creature if at least one of your allies is within 5 feet of the creature and the ally isn’t incapacitated.
Nimble Escape: The Skaven can take the Disengage or Hide action as a Bonus Action on each of its turns.
Two-Weapon Fighting: When the Night Runner engages in two-weapon fighting, it adds it's DEX modifier to the damage of the second Attack.
Pounce: If the Night Runner moves at least 20 ft. straight toward a creature and then hits it with a shortsword attack on the same turn, that target must succeed on a DC 12 Strength saving throw or be knocked prone.
Multiattack: The Night Runner makes two melee attacks with its shortswords, and may make one additional melee attack with its bonus action.
Shortswords: +6, 8(1d6+3[+1d4 poison])
Shortbow: +6, 8(1d6+3[1d4 poison]), 80/320 ft.

Poison-Wind Globadier
AC: 12 (robes)
HP: 33 (6d6+12)
Speed: 30 ft.
STR 10 (0) DEX 14 (+2) CON 14 (+2)
INT 14 (+2) WIS 12 (+1) CHA 8 (-1)
Sunlight Sensitivity: While in sunlight, the Skaven has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Keen Smell: The Skaven has advantage on Wisdom (Perception) checks that rely on smell.
Pack Tactics: The Skaven has advantage on an attack roll against a creature if at least one of your allies is within 5 feet of the creature and the ally isn’t incapacitated.
Poison Resistance: The Skaven has advantage on saving throws against poison and disease, and is resistant to poison damage.
Poison-Wind Globe: The Globe creates a 20-foot-radius sphere of yellow, nauseating gas centered on a point within 30 ft. The cloud spreads around corners, and its area is heavily obscured. The cloud lingers in the air for 3 rounds. Each creature that is completely within the cloud at the start of its turn must make a Constitution saving throw against poison. On a failed save, the creature spends its action that turn retching and reeling. Creatures that don't need to breathe or are immune to poison automatically succeed on this saving throw. A moderate wind (at least 10 miles per hour) disperses the cloud after 2 rounds. A strong wind (at least 20 miles per hour) disperses it after 1 round. DC 13, recharge 5-6.
Dagger: +5, 4(1d4+2)

Plague Priest
AC: 14 (hide armor)
HP: 55 (10d6+20)
Speed: 30 ft.
STR 10 (0) DEX 14 (+2) CON 14 (+2)
INT 10 (0) WIS 16 (+3) CHA 10 (0)
Sunlight Sensitivity: While in sunlight, the Skaven has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Keen Smell: The Skaven has advantage on Wisdom (Perception) checks that rely on smell.
Pack Tactics: The Skaven has advantage on an attack roll against a creature if at least one of your allies is within 5 feet of the creature and the ally isn’t incapacitated.
Redirect Attack: When a creature the Skaven can see targets it with an attack, the Skaven chooses another Skaven within 5 feet of it. The two Skaven swap places, and the chosen Skaven becomes the target instead.
Poison Resistance: The Skaven has advantage on saving throws against poison and disease, and is resistant to poison damage.
Divine Strike: Once on each of the Plague Priest's turns when it hits a creature with a weapon Attack, it can cause the Attack to deal an extra 4(1d8) poison damage to the target.
Scythe: +6, 11(1d10+2[+1d8 poison])
Spellcasting:
0: Infestation, Poison Spray, Thaumaturgy
1: Bane, Ray of Sickness
2: Blindness/Deafness, Ray of Enfeeblement
3: Bestow Curse, Stinking Cloud
4: Blight, Sickening Radiance
5: Cloudkill, Contagion

Plague Monk
AC: 16 (Unarmored Defense)
HP: 39 (6d6+18)
Speed: 30 ft.
STR 10 (0) DEX 16 (+3) CON 16 (+3)
INT 8 (0) WIS 12 (+1) CHA 8 (0)
Sunlight Sensitivity: While in sunlight, the Skaven has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Keen Smell: The Skaven has advantage on Wisdom (Perception) checks that rely on smell.
Pack Tactics: The Skaven has advantage on an attack roll against a creature if at least one of your allies is within 5 feet of the creature and the ally isn’t incapacitated.
Aggressive: As a bonus action, the Skaven can move up to its speed toward a hostile creature that it can see.
Unarmored Defense: While the Skaven is not wearing any armor, its Armor Class equals 10 + its Dexterity modifier + its Constitution modifier.
Relentless Endurance: When the Skaven is reduced to 0 Hit Points, it can drop to 1 hit point instead. The Skaven can't use this feature again until it finishes a long rest.
Poison Resistance: The Skaven has advantage on saving throws against poison and disease, and is resistant to poison damage.
Two Weapon Fighting: When the Plague Monk engages in two-weapon fighting, it adds it's DEX modifier to the damage of the second Attack.
Multiattack: The Night Runner makes two melee attacks with its scimitars, and may make one additional melee attack with its bonus action.
Scimitars: +6, 8(1d6+3[+1d4 poison])
Bite: +6, 5(1d4+3)

The Skaven Slaves form the foundation upon which I built all the others. Each type builds on that framework with new abilities and better equipment to fit their place in the army. The Slaves charge forward and swarm their foes, dying in droves. The Clanrats fight behind them with their reach weapons (personal house rule spears are finesse and reach, lose thrown). Stormvermin form the elite reserve, hacking down the tougher foes through the horde. Night Runners skirt the edge of the fight pouncing on squishy targets. Globadiers hurl their poison without much regard for collateral damage to the Slaves. Priests lead the army and cripple their foes so their weak underlings can bring them down. Monks are berserk warriors that shrug off fatal blows in their mad fervor.

There are a few elements that I'm still flexible on. I might increase the hit die size of Stormvermin and Plague Monks if they need to be tougher. Poison Resistance on Globardiers, Priests and Monks could possibly be upgraded to immunity, but I'm not sure that fits. It doesn't entirely fit their lore since Monks in particular intentionally infect themselves with disease.

I'm not sure how to calculate CR, or just how dangerous a horde of these guys can be with advantage being so prevalent even amongst the lowliest Slaves, but I'm building off of the Goblin and Kobold chassis so I hope it's not too crazy.

Thoughts, suggestions and advice would be much appreciated!

[Composer99]

As Hit Die size (d4, d6, etc.) is a function of a creature's size in 5e, if you think some of these need more hit points, you'll need to give them more Hit Dice, or make them Medium sized instead of Small.

I see none of the features are fleshed out. That's fine for well-known features, such as Pack Tactics, but what does, say, Cowardly Wretch do? (Maybe there's a monster in the MM that has that feature, but the point of these statblocks is that you shouldn't have to go hunting through the MM for feature descriptions - the only "external reference" is usually spell descriptions.) For the Skaven with Multiattack, how many attacks do they get, and with which weapons?

As a final nitpick, it's my understanding that if the only thing modifying a monster's AC is its Dexterity modifier, you don't specify the "armour" it's wearing (e.g. the guys with loincloths).

Slave rats have, according to the rough-and-dirty CR calculations in the DMG, a CR of 1/2. Since they're obviously the closest to goblins, I assume the target CR is 1/4? (That said, using those guidelines I think a goblin could end up CR 1/2 as well...) If so, I'd consider ditching Pack Tactics. Actually, I'd consider ditching it for Slave rats, period. Being good enough at fighting to get a solid edge when fighting together just doesn't strike me as a Slave rat "thing". Stormvermin? Definitely. Clan rats? Sure, why not? Slave rats? ... Egh, don't really buy it.

I'm nodding off at the keyboard, so that's it for now.

[ZenBear]

As Hit Die size (d4, d6, etc.) is a function of a creature's size in 5e, if you think some of these need more hit points, you'll need to give them more Hit Dice, or make them Medium sized instead of Small.

I see none of the features are fleshed out. That's fine for well-known features, such as Pack Tactics, but what does, say, Cowardly Wretch do? (Maybe there's a monster in the MM that has that feature, but the point of these statblocks is that you shouldn't have to go hunting through the MM for feature descriptions - the only "external reference" is usually spell descriptions.) For the Skaven with Multiattack, how many attacks do they get, and with which weapons?

As a final nitpick, it's my understanding that if the only thing modifying a monster's AC is its Dexterity modifier, you don't specify the "armour" it's wearing (e.g. the guys with loincloths).

Slave rats have, according to the rough-and-dirty CR calculations in the DMG, a CR of 1/2. Since they're obviously the closest to goblins, I assume the target CR is 1/4? (That said, using those guidelines I think a goblin could end up CR 1/2 as well...) If so, I'd consider ditching Pack Tactics. Actually, I'd consider ditching it for Slave rats, period. Being good enough at fighting to get a solid edge when fighting together just doesn't strike me as a Slave rat "thing". Stormvermin? Definitely. Clan rats? Sure, why not? Slave rats? ... Egh, don't really buy it.

I'm nodding off at the keyboard, so that's it for now.

Thanks for the tips! I was also leery of giving the Slaves Pack Tactics, but I thought it would be okay since Kobolds had it. Still, I think it's better that they don't have it so I took it out. I also removed Nimble Escape from everyone but the Night Runners since it fits them best. I fleshed out all the abilities too. I didn't at first because I'm lazy and I did it on my phone during breaks at work.

[Sir Brett Nortj]

I remember building armies for my Taurus character player once, was a mission, but it paid off in the end. Was quite an army, that never got to fight...

He was a ninth level second edition minotaur that was in the dark sun campaign, had driders and muls and all sorts of wonderful things up his sleeve.

Then, I gave him a soul sucking sword, to charge up on paladins and holy folk. Each time to killed a good creature, you got a charge, and, the adventure followed to find evil warlocks to decipher the sword's powers, s he learned them, he used the souls like spell points.

Yes, he was a warrior mage. Yes, I had a player character with him.

But, nice to see some 'wars!'

[ZenBear]

I’m trying to scale down the Stormvermin to be CR 1 or 2 to replace the bugbears under Tresendar Manor, and I would appreciate some feedback.

Stormvermin
AC: 16 (scale mail)
HP: 27 (5d6+10)
Speed: 30 ft.
STR 14 (+2) DEX 14 (+2) CON 14 (+2)
INT 10 (0) WIS 12 (+1) CHA 10 (0)
Sunlight Sensitivity
Keen Smell
Pack Tactics
Martial Advantage
Glaive: +4, 7 (1d10+2), reach
Crossbow, Heavy: +4, 7 (1d10+2), 100/400 ft.
CR: 1?

According to the DMG their HP is in the CR 1/8 range, and AC 16 is 3 above the expected 13 so that upgrades its Defensive CR to 1/4. His DPR is 14 which is CR 1 and attack bonus is 1 higher than the expected +3 so that bumps it up to Offensive CR 2. Then Pack Tactics bumps it once more to OCR 3. The average of DCR 1/4 and OCR 3 is effective CR 1. Did I do this right?

[ZenBear]

I've updated my Plague Monk and Plague Priest to bring them into the next encounter. I'm using the very helpful guide Monster Building 101 by The Angry DM to help me with this process. I'm aiming for CR 1 and 3 respectively, but I'm struggling to figure out how much HP my Priest should have.

Plague Monk
AC: 12
HP: 27 (5d6+10)
Speed: 35 ft.
STR: 10 (0) DEX: 14 (+2) CON: 14 (+2)
INT: 8 (-1) WIS: 12 (+1) CHA: 8 (-1)
Skills: Perception +3, Stealth +4
Sunlight Sensitivity
Keen Smell
Pack Tactics
Relentless
Pestilent: Devotees of Clan Pestilens are immune to the effects of poison and disease, though they remain capable of carrying such afflictions.
Actions:
Multiattack: The Plague Monk makes two attacks with its fetid blades.
Fetid Blades: +4 to hit, 5 (1d6+2) slashing damage, and the target must make a DC 10 CON saving throw, taking 2 (1d4) poison damage on a failed save, or half as much on a success.

So 27 HP and 12 AC falls in the Defensive CR 1/8 category, even when factoring in Relentless which says to add 7 effective HP bringing it up to 34. It deals a max of 14 damage per turn landing just barely within Offesive CR 1. Pack Tactics adjusts the attack bonus from +4 to +5, which is 2 higher than the expected +3 for CR 1 bumping it up to OCR 2. Add them both together gets CR 2 1/8, divide by 2 is 1 1/16 which I'm technically supposed to round up, but 1/16 seems like a negligible amount so this should still be Effective CR 1.

Plague Priest
AC: 14 (hide armor)
HP: 97 (15d6+45)
Speed: 35 ft.
STR: 10 (0) DEX: 14 (+2) CON: 16 (+3)
INT: 10 (0) WIS: 16 (+3) CHA: 10 (0)
Skills: Perception +6, Stealth +5
Sunlight Sensitivity
Keen Smell
Pack Tactics
Pestilent
Redirect Attack
Actions:
Multiattack: The Plague Monk makes two attacks with its Plague Censer.
Plague Censer: +4 to hit, reach 10 ft., 7 (1d10+2) bludgeoning damage, and the target must make a DC 13 CON saving throw or contract a disease. Until the disease is cured, the target can't regain hit points except by magical means, and the target's hit point maximum decreases by 3 (1d6) every 24 hours. If the target's hit point maximum drops to 0 as a result of this disease, the target dies.
Spellcasting:
0: Infestation, Spare the Dying, Thaumaturgy
1: Bane, Ray of Sickness
2: Blindness/Deafness, Ray of Enfeeblement
3: Bestow Curse, Stinking Cloud

Spellcasting is really hard to calculate for. The spells it has don't do much damage, the worst being Ray of Sickness which only does 8, so Offesive CR is currently in the 1/2 range, adjusted to 1 by the +5 attack bonus and Pack Tactics. That would mean I need the Defensive CR to be at 5, which seems absurd considering that HP range is 131-145 with AC 15. I'm all for having tough baddies that require more than a single round to bring down with focused fire, but should I really be aiming that high? Considering Skaven are Small creatures, that would mean I would need 24d6+48 to reach 132 HP. Again, this just seems silly. How would I take into account all the myriad ways the Priest can debuff the players? I intend for the encounter to consist of 2 Monks and 1 Priest, and they're all immune to the effects of Stinking Cloud other than the line of sight block, which is just going to make the Priest harder to kill. It also seems silly that the Monks would only have 27 HP as front line fighters but their caster would have 5 times that. Perhaps if I give it more damage on its Plague Censer, maybe Multiattack or Divine Strike, it would raise the OCR enough to warrant a lesser DCR? Then again, I like having combats take a little longer so the caster gets more time to use multiple spells, but I don't know. I could really use some suggestions.

For those who are curious, I got the Fetid Blade poison damage value from the Basic Poison and the Plague Censer effect from the Giant Rat (Diseased).

Edit: Decided to reduce the CR to 2 by bumping OCR and DCR into 2 range with Multiattack and 16 CON. Game is going down tonight, gotta make my choice.

[JNAProductions]

I’m trying to scale down the Stormvermin to be CR 1 or 2 to replace the bugbears under Tresendar Manor, and I would appreciate some feedback.

Stormvermin
AC: 16 (scale mail)
HP: 27 (5d6+10)
Speed: 30 ft.
STR 14 (+2) DEX 14 (+2) CON 14 (+2)
INT 10 (0) WIS 12 (+1) CHA 10 (0)
Sunlight Sensitivity
Keen Smell
Pack Tactics
Martial Advantage
Glaive: +4, 7 (1d10+2), reach
Crossbow, Heavy: +4, 7 (1d10+2), 100/400 ft.
CR: 1?

According to the DMG their HP is in the CR 1/8 range, and AC 16 is 3 above the expected 13 so that upgrades its Defensive CR to 1/4. His DPR is 14 which is CR 1 and attack bonus is 1 higher than the expected +3 so that bumps it up to Offensive CR 2. Then Pack Tactics bumps it once more to OCR 3. The average of DCR 1/4 and OCR 3 is effective CR 1. Did I do this right?

Not quite. You need a 2 point attack difference to up the OCR by 1.

[ZenBear]

Not quite. You need a 2 point attack difference to up the OCR by 1.

Thanks for the reply JNA! Good catch. Let’s try again;

Stormvermin
AC: 16 (scale)
HP: 44 (8d6+16)
Speed: 30 ft.
STR: 14 (+3) DEX: 14 (+2) CON: 14 (+2)
INT: 10 (0) WIS: 12 (+1) CHA: 10 (0)
Sunlight Sensitivity
Keen Smell
Pack Tactics
Martial Advantage: 7 (2d6)
Glaive: +4, 7 (1d10+2), reach
CR: 2

I’m scaling them up just a bit since they are supposed to be very scary elite troops. I’ll also be adding a slightly tougher stat block for the leader;

Clan Chief
AC: 18 (scale+shield)
HP: 55 (10d6+10)
Speed: 30 ft.
STR: 16 (+3) DEX: 14 (+2) CON: 14 (+2)
INT: 12 (+1) WIS: 12 (+1) CHA: 14 (+2)
Sunlight Sensitivity
Keen Smell
Pack Tactics
Martial Advantage: 7 (2d6)
Multiattack: The Clan Chief makes two longsword attacks.
Longsword: +5, 7 (1d8+3)
CR: 3

My players will be clearing Cragmaw Castle next session, and in addition to the Stormvermin, Plague Monks and Plague Priest they will be encountering in various rooms, I will be replacing the Owlbear with the Rat Ogre.

Rat Ogre
AC: 14 (natural armor)
HP: 126 (12d10+60)
Speed: 40 ft.
STR: 20 (+5) DEX: 10 (0) CON: 20 (+5)
INT: 5 (-3) WIS: 9 (-1) CHA: 6 (-2)
Sunlight Sensitivity
Keen Smell
Squash: +8, 26 (6d6+5), the Rat Ogre lands prone in the target’s space, and the target is grappled (escape DC 15). Until this grapple ends, the target is prone. The grapple ends early if the Rat Ogre stands up.
Multiattack: The Rat Ogre makes two Slam attacks.
Slam: +8, 18 (3d8+5), 5ft range. If the Rat Ogre hits the same target with both Slam attacks, after resolving the attacks it may immediately attempt to Shove the target.
CR: 5

This is essentially a reskin of the Hill Giant, because that Squash feature is hilarious and very on-point for the Rat Ogres as they do exactly this kind of flying belly flop attack on a charge in Total War: Warhammer. My players will be level 4, but they have previously taken down a Shambling Mound and Will o’ Wisp combo at level 3 so I’m not worried. I also added the Shove when both Slam attacks hit to simulate the way the Rat Ogre throws players around in Warhammer: Vermintide. I would prefer if it could throw them a lot further than 5 ft., though, so perhaps I’ll change that up a bit. As it is now, it’s most useful in knocking foes prone.

Once my players clear Cragmaw, they’ll be moving on to Wave Echo Cave, and while they’ll be meeting new enemy types like the Drow and Undead, I will be replacing the boring Ochre Jelly encounter with their first Night Runner.

Night Runner
AC: 14 (leather)
HP: 27 (6d6+6)
Speed: 35 ft.
STR: 10 (0) DEX: 16 (+3) CON: 12 (+1)
INT: 12 (+1) WIS: 12 (+1) CHA: 8 (-1)
Skills: Stealth +9 (Expertise)
Sunlight Sensitivity
Keen Smell
Pack Tactics
Ambusher: In the first round of a combat, the Night Runner has advantage on attack rolls against any creature it surprised.
Surprise Attack: If the Night Runner surprises a creature and hits it with an attack during the first round of combat, the target takes an extra 10 (3d6) damage from the attack.
Multiattack: The Night Runner makes two short sword attacks.
Short Sword: +6, 6 (1d6+3)
Shuriken: +6, 5 (1d4+3), 20/60 ft.
CR: 3

As seen above, I originally intended the Night Runners to have Nimble Escape, but apparently that has a massive effect on CR and I couldn’t find a comfortable way to average that out to reach the CR I want. Since this is supposed to replace the Ochre Jelly I originally intended it to be CR 2, but when I saw the Doppelganger stat block I realized Ambusher/Surprise Attack fit the bill better and since my party is so damn powerful CR 3 shouldn’t be much trouble for them. If the Wizard gets caught out he’ll be down in the surprise round, but the Paladin has healing for days so it should be fine. I’ll save Nimble Escape for the higher ranks Clan Eshin rats like the Gutter Runners and Assassins.

[ZenBear]

Got inspired to make another Skaven to replace the Flaming Skull in Wave Echo Cave. In my game the smelter will be currently in use by the Skaven to forge ammunition and construction materials for the shipwrights in the Deadmines. I think such a set up requires someone with a bit of technological expertise, so I decided to stat up a Warlock-Engineer.

Warlock-Engineer
AC: 16 (scale mail) HP: 55 (10d6+20) Speed: 35 ft.
STR: 14 (+2) DEX: 14 (+2) CON: 14 (+2)
INT: 16 (+3) WIS: 12 (+1) CHA: 10 (0)
Skills: Arcana +5
Sunlight Sensitivity: While in sunlight, the Skaven has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Keen Smell: The Skaven has advantage on Wisdom (Perception) checks that rely on smell.
Pack Tactics: The Skaven has advantage on an attack roll against a creature if at least one of your allies is within 5 feet of the creature and the ally isn’t incapacitated.
Redirect Attack: When a creature the Skaven can see targets it with an attack, the Skaven chooses another Skaven within 5 feet of it. The two Skaven swap places, and the chosen Skaven becomes the target instead.
Spellcasting: The Warlock-Engineer is a 5th level caster. It’s spellcasting ability is Intelligence (spell save DC 14, +6 to hit with spell attacks). It knows the following spells:
Cantrips (at-will): Mending, Shocking Grasp
1st level (3 slots) : Thunderous Smite, Warp Armor (Armor of Agathys, lighting damage)
2nd level (3 slots): Scorch (Aganazzar's Scorcher), Skitterleap (Misty Step)
3rd level (2 slot): Death Frenzy (Crusader’s Mantle), Warp Lightning (Call Lightning)
Actions:
Multiattack: The Warlock-Engineer makes two attacks with its glaive.
Glaive: +5 to hit, reach 10 ft., 7 (1d10+2) slashing damage.
CR: 3

DCR is 1/2 with that HP, boosted to 1 by the AC. OCR would be 4 with Scorch dealing 27 damage and Warp Lightning dealing 32 if they hit 2 targets, but if Scorch is upcast at 3rd level it comes out to 36, so OCR is actually 5. That makes the ECR 3. I intend the encounter in the smelting chamber to be the Warlock-Engineer and 6-8 skaven slaves, so first round it will cast Death Frenzy to make them a credible threat as 4 charge forward to jam up the party and the rest fire with their slings. The Wizard will likely cast Burning Hands, as he is wont to do, which will surely kill the 4 front line rats and injure the W-E if he's in range, so at best they'll be a one-round speed bump if at all. Having them charge in two waves should help, and dealing bonus damage on their slings with Death Frenzy will make things interesting.

I've also slightly tweaked the Slaves and Clanrats:

Skaven Slave
AC: 12
HP: 5 (2d6-2)
Speed: 35 ft.
STR 10 (0) DEX 14 (+2) CON 8 (-1)
INT 8 (-1) WIS 12 (+1) CHA 6 (-2)
Sunlight Sensitivity: While in sunlight, the Skaven has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Keen Smell: The Skaven has advantage on Wisdom (Perception) checks that rely on smell.
Cowardly Wretch: The Skaven Slave has disadvantage on saving throws against being frightened unless lead by more a powerful Skaven.
Dagger: +4, 4(1d4+2) piercing damage.
Sling: +4, 4(1d4+2) bludgeoning damage, 30/120 ft.
Bite: +4, 4(1d4+2) piercing damage.
CR: 1/8

Clanrat
AC: 16 (hide armor & shield)
HP: 20 (4d6+4)
Speed: 35 ft.
STR 12 (+1) DEX 14 (+2) CON 12 (+1)
INT 10 (0) WIS 12 (+1) CHA 8 (-1)
Sunlight Sensitivity: While in sunlight, the Skaven has disadvantage on Attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Keen Smell: The Skaven has advantage on Wisdom (Perception) checks that rely on smell.
Pack Tactics: The Skaven has advantage on an attack roll against a creature if at least one of your allies is within 5 feet of the creature and the ally isn’t incapacitated.
Shortsword: +4, 5(1d6+2) piercing damage.
Bite: +4, 4(1d4+2) piercing damage.
CR: 1/4