Archlich

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

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

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

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

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

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

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

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, plane shift, teleport

8th level (2 slots): maze, mind blank,

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

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

Paralyzing Touch. Melee Spell Attack: +8 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.

The 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. The archlich regains spent legendary actions at the start of his turn.

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

Melee Attack. The archlich uses Paralyzing Touch 

Frightening Gaze (Costs 2 Actions). The archlich 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 all archlichs gaze for the next 24 hours. 

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