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. Finally, Acererak has obtained a ring of spell turning.

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

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. 

Resistance of the Grave. Acererak is immune against any effect that turns undead.

Paralyzing Touch. Melee Spell Attack: +18 to hit, reach 5 ft., one creature. Hit: 18 (4d8) cold damage, and the target must succeed on a DC 26 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. For every round a creature is paralyzed, the saving throw DC is reduced by 3.

Staff. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage plus 10 (3d6) necrotic damage, or 7 (1d8 + 3) 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 26 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 spell ends the curse on the target.

Spellcasting. Acererak casts one of the following spells, requiring no material components and using Intelligence as the spellcasting ability (spell save DC 26):

At will: animate dead (as an action), detect magic, dispel magic, fireball, slow, mage hand, prestidigitation

2/day each: abi-dalzim's horrid wilting, finger of death, harm

1/day each: time ravage, dominate monster, tether essence

Spell Surge (3/day): Acererak can cast a spell that has a normal casting time of one Action as a Bonus Action instead. Acererak can only use their action to cast an At Will spell after using this Bonus Action.

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.

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.

Cast a Spell (Costs 3 Actions). Acererak casts a spell that is not "at-will".

Disrupt Life (Costs 3 Actions). Each creature within 30 feet of Acererak must make a DC 21 Constitution saving throw against this magic, taking 42 (12d6) necrotic damage on a failed save, or half as much damage on a successful one.