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.

Paralyzing Aura. A creature that damages the demilich with a melee attack must succeed on a DC 20 Constitution saving throw or be Paralyzed until the end of the demilich's next turn, or until the creature takes damage.

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

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

Multiattack. The demilich has four attacks (telekenis or sould wrench). It can replace one attack with Wrench Heart or Disintegrate.

Baleful Telekinesis. One creature within 60 feet that the demilich can see must succeed on a DC 20 Strength saving throw or be moved 30 feet. If this movement carries the creature into a wall or other solid surface, the target takes 33 (6d10) bludgeoning damage.

Soul Sap. Ranged spell attack. +13 to hit. On a hit target takes 4d6+7 necrotic damage and an additional 2d6 necrotic damage for every level of exhaustion.

Wrench Heart (7th Level Spell, Concentration recharge 6). One creature within 60 feet that the demilich can see feels a spectral hand sink into its chest and grip its heart. At the begining of each of the creature's turns it takes 45 (10d8) necrotic damage and must succeed on a DC 20 Wisdom saving throw or be Frightened until the beginning of its next turn.

Disintegrate (6th Level Spell, (Recharge 5-6). One creature within 60 feet must succeed on a DC 20 Dexterity saving throw or take 80 (10d6 + 45) force damage, or half as much on a success. If this damage reduces a creature to 0 hit points, they are disintigrated.

Brain Shiver. Target one creature within 60 feet. Target must make a DC 15 save with their lowest mental stat. On a fail the target must subtract 1d4+1 from their next d20 test. If The target subjects Acererak to a save before they make a d20 test Acererak may instead add 1d4+1 to this save instead.

Shield (1st Level Spell). As a reaction to being targeted with an attack, the demilich may add 5 to its AC until the beginning of its next turn.

Spelldrain Cackle. As a reaction to seeing a creature cast a spell within 60 feet, the demilich forces the spell caster to succeed on a DC 18 Charisma saving throw or have the spell fail and its highest level unexpended spell slot drained.

Howl (1/Day). As a reaction to being reduced below 100 hit points, the demilich emits a bloodcurdling howl. Each creature within 30 feet that can hear the howl must succeed on a DC 20 Constitution saving throw or drop to 0 hit points. A creature that fails by 5 or more dies.

Acererak can take 4 Legendary actions

Move. (costs 1) Acererak moves up to 50 feet without provoking attacks

Chatter. (costs 1) Each creature within 60 feet that is concentrating and can hear the demilich must succeed on a DC 20 Constitution saving throw or lose concentration.

Attack. (costs 1) Acererak makes one attack with soul wrench or telekinesis  

Trap Soul. (costs 4) The demilich targets one creature that it can see within 30 feet of it. The target must make a DC 18 Charisma saving throw. On a failed save, the target’s soul is magically trapped inside one of the demilich’s gems. While the soul is trapped, the target’s body and all the equipment it is carrying cease to exist. On a successful save, the target takes 24 (7d6) necrotic damage, and if this damage reduces the target to 0 hit points, its soul is trapped as if it failed the saving throw. A soul trapped in a gem for 24 hours is devoured and ceases to exist.

If the demilich drops to 0 hit points, it is destroyed and turns to powder, leaving behind its gems. Crushing a gem releases any soul trapped within, at which point the target’s body re-forms in an unoccupied space nearest to the gem and in the same state as when it was trapped.