D&D 5E (2014) Help Me Create an Epic Final Boss Fight!

[jayoungr]

Hi, folks! A two-year campaign is coming to a close in a couple of days, and I could use some help making the final fight extra special.

The party is level 8 (bard, warlock, fighter, barbarian, rogue, and a custom class shaman). They will be fighting their way through a mist-infested abandoned village to face down the evil spirit that has cursed the place. The spirit is bound to a cracked clay vessel and cannot move (much) from its place, but its influence covers the whole territory. Their object will be to weaken the spirit enough to force it back into the vessel and then use mending to repair the vessel.

More useful information about the setup:

• The evil spirit's powers have been established as follows:
  • Mist causes illness (but this should be fairly slow-acting)
  • Voices from the mist can lure the unwary
  • The mist is full of hostile shapes, either spawned by or part of the boss (no one is quite sure which)
• The party includes a glamour bard who has no offensive spells but is our primary healer. The bard likes to spam mantle of inspiration for temporary hitpoints and free movement, plus is also the only character in the party to have calm emotions.
• The warlock has a necklace of fireballs that she's itching to use.
• I have a custom map which is quite large, so it should take the party a few rounds to reach the boss.
  • Thinking of having low-level enemies spawn every round to slow the party down and soak up some of those fireball charges.
• Legendary actions and lair actions are desirable.

Help me figure out how to make this an epic fight!

I'm thinking of a lair action requiring a wisdom save to avoid being charmed by voices from the mist. Maybe another one with a save to avoid illness, but I'm not sure how to represent failure; the poisoned condition would be too strong in this case. Waves of enemies should probably spawn each round, but how many and what level? I could reskin something as a mistwraith, but I'm also not sure what base monster to start with.
Any suggestions??

 

[Storyteller Hero]

The animated corpses of the original villagers

Mirrors on the walls that the spirit can use to teleport between locations

NOTE: Does the party have a plan to make sure the spirit is not able to simply run away? One easy idea is that the spirit is afraid of sunlight because they were originally a killer executed in daylight by being swarmed with ants while tied to a pole, fearing the dawn since that was when the ants came out of their burrows.

 

[Ulorian - Agent of Chaos]

• The warlock has a necklace of fireballs that she's itching to use.

It would be cool to slip in a horde of mooks that could be wiped with a fireball. If this group was a later round introduction, I could see the warlock dropping a fireball on this crowd making for a memorable moment. Even if accompanied by a heftier individual or two; the mooks get incinerated, the leaders are still there, but suitably weakened and demoralised, their surprise attack thwarted... good stuff.

But of course, if they show up, and aren't whittled down by the fireball(s), your fight scene gets increased in difficulty tremendously. But that's part of the fun honestly...

 

[jayoungr]

NOTE: Does the party have a plan to make sure the spirit is not able to simply run away?

Yes, they have information about the spirit's past and how it was bound to the vessel.

 

[J-H]

Where did the spirit come from? What are its themes beyond spooky and causes disease and fog?
I know you want a boss fight, but this sounds more like undifferentiated manifestations of its presence rather than a single boss.

Here's a Poison Mist elemental. It can be nearly invisible in the fog, so it makes a good surprise attacker, and it seems like it's on-theme as something they encounter.

Spoiler: Poison Mist CR 7

Poison Mist
A grey-green form swirls in the mist, reaching forwards with two insubstantial arms. A brown cloud seems to swirl around it, obscuring its precise position.
Medium Elemental
Armor Class 14
Hit Points 90
Speed 30’ fly
Str Dex Con Int Wis Cha
6(-1) 18(+4) 16(+3) 6(-2) 14(+2) 8(-1)
Skills Stealth +7
Resistances acid, cold, necrotic
Damage Immunities Non-magical weapon damage, Poison
Vulnerabilities thunder
Condition Immunities Charmed, Exhaustion, Grappled, Frightened, Paralyzed, Poisoned, Prone, Restrained, Unconscious
Senses Blindsense 60’, passive Perception 12
CR 7
Blur Attack rolls against the Poison Mist suffer disadvantage, unless the attacker has a way to sense invisible enemies.
Poison Aura Any living creature who starts its turn within 10’ of the Poison Mist takes 2d12 poison damage and is Poisoned for one round. A DC 14 Constitution save halves the damage and negates the Poisoned condition.
Actions
Multiattack. The Poison Mist attacks twice, using any of its melee or ranged attack options.
Dissolving Touch. Melee Weapon Attack, +7 to hit, 5’, 2d6 acid damage. On a successful hit, the next attack against the target has advantage.
Gaseous Swipe. Melee Weapon Attack, +7 to hit, 5’, 3d6 poison damage.
Poison Bubble. A target within 30’ must make a DC 14 Constitution save or take 2d12 poison damage.

 

[Rabulias]

Maybe failing the saving throw against the poison gives disadvantage on Wisdom saving throws?
And failing a Wisdom saving throw makes one see illusionary opponents (or friends?) within the mist that lead the PCs away from the boss, or maybe they entice the PCs into breaking the clay vessel.