Special Equipment. Zadar carries the Staff of the Forgotten One in one hand, and Sword of Kas in the other. 

Legendary Resistance (4/Day). If Zadar fails a saving throw, he can choose to succeed instead.

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

Life Drain. Using the Sword of Kas to cast contagion Zadar can heal the amount of hit points that the poison deals to the target.

Spellcasting. Zadar is a 20th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 23, +15 to hit with spell attacks). Zadar has the following wizard spells prepared:

Cantrips (at will): mage hand, ray of frost, shocking grasp

1st level (at will): ray of sickness, shield

2nd level (at will): arcane lock, knock

3rd level (at will): animate dead, counterspell

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

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

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

7th level (3 slots): finger of death, fire storm, teleport

8th level (2 slots): maze, antimagic field

9th level (2 slots): psychic scream, prismatic wall

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

Spell Resistance.  Zadar has advantage on saving throws against any magical effects and spell attacks and neither he nor his weapons are affected be his anitmagic field.

Multiattack. Zadar can use either Paralyzing Touch, his staff, or Invoke Curse, or cast a spell. Then he can attack once with his sword.

Paralyzing Touch. Melee Spell Attack: +10 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.

Sword. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 7 (2d8 + 4) bludgeoning damage plus 10 (3d6) necrotic damage.

Infect. Every time Zadar hits a target with Sword of Kas he can choose to cast contagion on that target (3 Charges). Along with being poisoned, the target must take (2d6 + 6) poison damage. Zadar heals for the amount of poison damage dealt. Once the target is no longer poisoned they can't be infected again.

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

Invoke Curse. While holding the Staff of the Forgotten One, Zadar expends 1 charge from it and targets one creature he can see within 60 feet of him. The target must succeed on a DC 23 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.

Vile Teleport. Zadar teleports, along with any equipment he is wearing or carrying, up to 60 feet to an unoccupied space he can see. Each creature within 15 feet of his destination space must make a DC 18 Strength Save, if failed the target gets pushed 15 feet away from Zadar, is knocked prone, and takes 6 (2d6) psychic damage, or takes half damage and isn't knocked back if successful. If at least one creature takes this damage, Zadar regains (6x the # of players within 15 feet) hit points.

Zadar can take 4 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. Zadar regains spent legendary actions at the start of his turn.

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

Melee Attack. Zadar uses Paralyzing Touch, makes one melee attack with his staff, or one melee attack with his sword.

Dread Counterspell. Zadar utters a dread word to interrupt a creature he can see that is casting a spell. If the spell is 4th level or lower, it fails and has no effect. If the spell is 5th level or higher, Zadar makes an Intelligence check (DC 10 + the spell’s level). On a success, the spell fails and has no effect. Whatever the spell’s level, the caster takes 10 (3d6) psychic damage if the spell fails.

Frightening Gaze (Costs 2 Actions). Zadar 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.

Disrupt Life (Costs 3 Actions). Each creature within 20 feet of Zadar 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.