Legendary Resistance (3/Day). If Acererak fails a saving throw, he can choose to succeed instead.

Lichdom. Acererak regains a number of hit points equal to any damage he dealt to a creature that isn't Undead or a Construct. If Acererak takes radiant damage from a good-aligned creature, this trait doesn't function until the end of his next turn.

Special Equipment. Acererak carries the Staff of the Forgotten One. In the hands of anyone other than Acererak, the Staff of the Forgotten One is a Staff of the Magi.

Spell Immunity. Acererak cannot take damage from 3rd level spells or lower. (This trait is bestowed by his Staff of the Forgotten One.)

Multiattack. Acererak makes two Staff of the Forgotten One attacks and one Paralyze attack.

Staff of the Forgotten One. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 14 (2d6 + 7) bludgeoning damage, or 16 (2d8 + 7) bludgeoning damage when used with two hands plus 10 (3d6) necrotic damage.

Paralyze. Ranged Spell Attack: +16 to hit, range 60/120 feet., one creature. Hit: the target is paralyzed until the end of its next turn.

Invoke Curse (Recharge 6). Acererak targets one creature he can see within 60 feet of himself. The target must succeed on a DC 23 Constitution saving throw or be cursed. While cursed in this way, the target cannot regain hit points and has vulnerability to necrotic damage. Greater Restoration, Remove Curse or similar magic ends the curse on the target.

Spellcasting. Acererak casts one of the following spells, requiring no material components and using Intelligence for his spellcasting ability (spell save DC 23):

At will: Arcane Lock, Blight, Chain Lightning, Circle of Death, Cloudkill, Dispel Magic, Hold Monster, Plane Shift, Project Image, Ray of Sickness, Shield

1/day each: Feeblemind, Finger of Death, Mass Suggestion, Maze, Time Stop

Misdirection. Acererak imposes disadvantage on a weapon attack roll that would hit him. If the attack misses, Acererak uses his Paralyze attack against the attacker.

Annihilating Counterspell. Acererak interrupts a creature he can see within 60 feet of himself that is casting a spell. If the spell is 5th level or lower, it fails and has no effect. If the spell is 6th level or higher, Acererak makes an Intelligence check (DC 10 plus the spell's level). On a successful check, the spell fails and has no effect. Whatever the spells level, the caster takes 13 (2d12) force damage. If this damage reduces a creature to 0 hit points, it is destroyed and nothing short of a wish spell can bring the creature back to life.

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

Staff. Acererak makes one Staff of the Forgotten One attack.

Teleport. Acererak magically teleports along with any equipment he is wearing or carrying, up to 120 feet to an unoccupied space he can see.

Cast a Spell (Costs 2 Actions). Acererak uses Spellcasting.

Disrupt Life (Costs 3 Actions). Each creature within 80 feet of Acererak must make a DC 23 Constitution saving throw, taking 42 (12d6) necrotic damage on a failed save, or half as much damage on a successful one.