Rejuvenation. If he has a phylactery, Acererak gains a new body in 1d10 days if destroyed, regaining all his hit points and becoming active again. The new body appears within 5 feet of the phylactery.

Spellcasting. Acererak is an 20th-level spellcaster. his spellcasting ability is Intelligence (spell save DC 24, +16 to hit with spell attacks). Acererak has the following wizard spells prepared:

• Cantrips (at will): mage hand, prestidigitation, ray of frost

• 1st level (4 slots): detect magic, magic missile, shield, thunderwave

• 2nd level (3 slots): detect thoughts, invisibility, Melf’s acid arrow, mirror image

• 3rd level (3 slots): animate dead, counterspell, dispel magic, fireball

• 4th level (3 slots): blight, dimension door

• 5th level (3 slots): cloudkill, scrying

• 6th level (1 slot): disintegrate, globe of invulnerability

• 7th level (1 slot): finger of death, plane shift

• 8th level (1 slot): dominate monster, power word stun

• 9th level (1 slot): power word kill

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

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

Turn Immunity. Acererak is immune to effects that turn undead.

Regeneration. Acererak regains 10 hit points at the start of each of his turns. If Acererak takes radiant or force damage, this trait doesn’t function at the start of Acererak’s next turn. Acererak dies only if he starts his turn with 0 hit points and doesn’t regenerate.

Howl (Recharge 5-6). Acererak emits a bloodcurdling howl. Each creature within 30 feet of Acererak that can hear the howl must succeed on a DC 20 Constitution saving throw or drop to 0 hit points. On a successful save, the creature is frightened until the end of its next turn.

Life Drain. Acererak targets up to three creatures that he can see within 10 feet of him. Each target must succeed on a DC 20 Constitution saving throw or take 21 (6d6) necrotic damage, and Acererak regains hit points equal to the total damage dealt to all targets.

Acererak 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. Acererak regains spent legendary actions at the start of his turn.

Flight. Acererak flies up to half its flying speed.

Cloud of Dust. Acererak magically swirls its dusty remains. Each creature within 10 feet of Acererak, including around a corner, must succeed on a DC 20 Constitution saving throw or be blinded until the end of Acererak’s next turn. A creature that succeeds on the saving throw is immune to this effect until the end of Acererak's next turn.

Energy Drain (Costs 2 Actions). Each creature with in 30 feet of Acererak must make a DC 20 Constitution saving throw. On a failed save, the creature’s hit point maximum is magically reduced by 10 (3d6). 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). Acererak targets one creature he can see within 30 feet of himself. The target must succeed on a DC 20 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.