Special Equipment. Acererak carries the Staff of the Forgotten One. He wears a talisman of the sphere and has a sphere of annihilation under his control.

Arcane Recovery (1/Day). When he finishes a short rest, Acererak recovers all his spell slots of 7th level and lower.

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

Magic Resistance. Acererak has advantage on saving throws against spells and other magical effects. Spell attack rolls against him are made at disadvantage.

Rejuvenation. Acererak’s body turns to dust when he drops to 0 hit points, and his equipment is left behind. Acererak gains a new body after 1d10 days, regaining all his hit points and becoming active again. The new body appears within 5 feet of Acererak’s phylactery, the location of which is hidden.

Spellcasting. Acererak is a 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): booming blade (3d8 + 4d8 if the target moves), chill touch (4d8), mage hand, message, ray of frost (4d8)

1st level (at will): chromatic orb, detect magic shield

2nd level (at will): acid arrow, blindness/deafness, detect thoughts

3rd level (at will): bestow curse, counterspell, fireball

4th level (3 slots): blight, ice storm, phantasmal killer

5th level (3 slots): enervation, hold monster, wall of force

6th level (3 slots): circle of death, disintegrate, eyebite

7th level (3 slots): finger of death, plane shift, teleport

8th level (2 slots): dominate monster, power word stun

9th level (2 slots): power word kill, time stop

Turn Resistance. Acererak has advantage on saving throws against any effect that turns undead.

Paralyzing Touch. Melee Spell Attack: +16 to hit, reach 5 ft., one creature. Hit: 10 (3d6) cold damage, and the target must succeed on a DC 20 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Staff. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) bludgeoning damage plus 10 (3d6) necrotic damage, or 8 (1d8 + 4) bludgeoning damage plus 10 (3d6) necrotic damage when used with two hands.

Invoke Curse. While holding the Staff of the Forgotten One, Acererak expends 1 charge from it and targets one creature he can see within 60 feet of him. The target must succeed on a DC 24 Constitution saving throw or be cursed. Until the curse is ended, the target can’t regain hit points and has vulnerability to necrotic damage. Greater Restoration, remove curse or similar magic ends the curse on the target.

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.

Howl (Recharge 5–6). Acererak emits a bloodcurdling howl. Each creature within 30 feet of him 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 it's next turn.

Acererak can take 5 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.

At-Will Spell. Acererak casts one of his at-will spells.

Melee Attack. Acererak uses Paralyzing Touch or makes one melee attack with his staff.

Frightening Gaze (Costs 2 Actions). Acererak fixes his gaze on one creature he can see within 10 feet of him. The target must succeed on a DC 20 Wisdom saving throw against this magic or become frightened for 1 minute. The frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a target's saving throw is successful or the effect ends on it, the target is immune to Acererak's gaze for the next 24 hours.

Talisman of the Sphere (Costs 2 Actions). Acererak uses his talisman of the sphere to move the sphere of annihilation under his control up to 90 feet.

Disrupt Life (Costs 3 Actions). Each creature within 20 feet of Acererak must make a DC 20 Constitution saving throw against this magic, taking 42 (12d6) necrotic damage on a failed save, or half as much damage on a successful one.

Energy Drain (Costs 2 Actions). Each creature within 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 him. 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.