Improving Acererak for a 6 Person Level 20 Party

[newsman77]

How would you improve Acererak to take on a party of 6 players, all 20th level?

My group wants to run Tomb of Horrors. At this level, I'm pretty sure they can blow through it. I'd like the last fight to at least be a challenge. If my math is right, he's coming in at a medium challenge.

As a newish DM, I'm a little unsure what to do. I had a few thoughts:

1) Add some minions. Maybe a few demons to bring up the encounter level to a deadly rating
2) Use a tier hp approach. Give Acererak at 3 seperate hp pools, only 1 of which can be depleted a round. That would allow me to put some cinematics in place.
3) Somehow have Acererak clone himself. This one bothers me because I don't know if Acereak would do this realistically. Althought the thought of having him split off into a few copies of himself would put the fear into the party.

What do you think? How could I make this a challenge for the party?

[Unoriginal]

How about using the Acererak's stats from Tomb of Annihilation, for starter?

He'll be strong enough to be a challenge to 4 lvl 20 PCs. So you just need a few more stuff to balance it out. Maybe golems.

[Ritorix]

I ran a lich fight recently against a campaign arch villain. If your group is anything like mine, by now they are a well-oiled machine and are going to burn through nearly anything.

I had the lich mystically linked to 3 allies - a fighter, paladin and wizard NPC. So it was a 4 vs 4 fight, but the 'boss' couldn't be hurt until all his allies were dead. If he took damage it was divided among the surviving allies instead.

The group immediately focused on the boss, of course, but once they realized who was taking damage they took down each of the linked NPCs. It worked out pretty well.

[Mith]

I ran a lich fight recently against a campaign arch villain. If your group is anything like mine, by now they are a well-oiled machine and are going to burn through nearly anything.

I had the lich mystically linked to 3 allies - a fighter, paladin and wizard NPC. So it was a 4 vs 4 fight, but the 'boss' couldn't be hurt until all his allies were dead. If he took damage it was divided among the surviving allies instead.

The group immediately focused on the boss, of course, but once they realized who was taking damage they took down each of the linked NPCs. It worked out pretty well.

A similar vein to this, I also like the idea of body snatching, where the boss steals the body of a minion, and gets their HP pool (or max HP, if it's higher).

[newsman77]

I ran a lich fight recently against a campaign arch villain. If your group is anything like mine, by now they are a well-oiled machine and are going to burn through nearly anything.

I had the lich mystically linked to 3 allies - a fighter, paladin and wizard NPC. So it was a 4 vs 4 fight, but the 'boss' couldn't be hurt until all his allies were dead. If he took damage it was divided among the surviving allies instead.

The group immediately focused on the boss, of course, but once they realized who was taking damage they took down each of the linked NPCs. It worked out pretty well.

I really like this idea. Thanks!

[Malifice]

How would you improve Acererak to take on a party of 6 players, all 20th level?

What do you think? How could I make this a challenge for the party?

Acerak
Tiny undead, neutral evil

Armor Class 25 (natural armor, magically enchanted gemstones)
Hit Points 200 (40d4+40)
Speed 0 ft., fly 30 ft. (hover)
STR 1 (-5 ) DEX 20 (+5) CON 12 (+1) INT 22 (+6) WIS 17 (+3)CHA 20 (+5)
Saving Throws Con +9, Int +14, Wis +11, Cha +13
Skills: Arcana + 14, Perception +11
Damage Resistances: bludgeoning, piercing, and slashing from magic weapons
Damage Immunities: necrotic, poison, psychic, bludgeoning, piercing, and slashing from nonmagical weapons
Condition Immunities: charmed, deafened, exhaustion, frightened, paralyzed, petrified, poisoned, prone, stunned
Senses: truesight 120 ft, Passive perception 21
Challenge 24

Avoidance. If the demilich is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Arcane Ward: Acerak has magically enchanted the gems that now for part of his body. He is immune to the Magic missile spell, gains a permanent +5 magical bonus to his AC, and is constantly under the effects of a Freedom of Movement spell.

Legendary Resistance (3/Day). If the demilich fails a saving throw, it can choose to succeed instead.

Turn Immunity. The demilich is immune to effects that turn undead.

Actions

Howl (Recharge 5-6). The demilich emits a bloodcurdling howl. Each creature within 30 feet of the demilich that can hear the howl must succeed on a DC 17 Constitution saving throw or drop to 0 hit points. On a successful save, the creature instead takes (22) 4d10 psychic damage and is frightened until the end of its next turn. Undead are immune to this effect.

Life Drain. The demilich targets up to three creatures that it can see within 10 feet of it. Each target must succeed on a DC 22 Constitution saving throw or take 35 (6d10) necrotic damage, and the demilich regains hit points equal to the total damage dealt to all targets.

Legendary Actions

The demilich can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time, and only at the end of another creature’s turn. The demilich regains spent legendary actions at the start of its turn.

Flight. The demilich flies up to half its flying speed, without provoking attacks of opportunity.

Cloud of Dust. The demilich magically swirls its dusty remains. Each creature within 10 feet of the demilich, including around a corner, must succeed on a DC 17 Constitution saving throw or be blinded until the end of the demilich’s next turn. A creature that succeeds on the saving throw is immune to this effect until the end of the demilich’s next turn.

Energy Drain (Costs 2 Actions). Each non undead creature with in 30 feet of the demilich must make a DC 17 Constitution saving throw. On a failed save, the creature’s hit point maximum is magically reduced by 16 (3d10). If a creature’s hit point maximum is reduced to 0 by this effect, the creature dies. A creature’s hit point maximum can be restored with the greater restoration spell or similar magic.

Vile Curse (Costs 3 Actions). The demilich targets one creature it can see within 30 feet of it. The target must succeed on a DC 17 Wisdom saving throw or be magically cursed. Until the curse ends, the target has disadvantage on attack rolls and saving throws. The target can repeat the saving throw at the end of each of its turns, ending the curse on a success.

For Lair actions, Aceraks antimagic field is always available on round 1, but otherwise as per the MM.

Tactics

On initiative count 20 of round 1, Acerak targets himself with his anti-magic field (this shuts down his cloud of dust, energy drain and vile curse legendary action abilities and removes his Arcane ward and reduces his AC to 20 while active, but he can still Howl and Life drain just fine in the field) and then, on his turn he moves to catch as many PCs as he can in the 30' radius and Howls.

Until initiative count 20 of round 2, he is immune to damage from weapons (seeing as all weapons that strike him are rendered non-magical in his AMF, and he is immune to non magic weapons) and cant be affected by spells. Magical flight cant be used to reach him either.

He uses his legendary action movement to position himself accordingly.

On the following turn, he uses a different lair action (pick one) and Howls again (if available), or uses life drain. If possible he readies an action to Howl (or Life drain) immediately following his Vile Curse legendary action this turn.

On turn 3 he re-establishes his AMF (on himself again) if its available on initiative count 20. and repeats turn 1-2.

Throw in some Spectre or Shadow mooks if you must (and your players can handle it).