Lich

If it has a phylactery, a destroyed lich gains a new body in 1d10 days, regaining all its hit points and becoming active again. The new body appears within 5 feet of the phylactery.

The lich is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). The lich has the following wizard spells prepared:
Cantrips (at will): mage hand, prestidigitation, ray of frost
1st level (4 slots): detect magic, magic missile, shield, thunderwave
2nd level (3 slots): detect thoughts, invisibility, acid arrow, mirror image
3rd level (3 slots): animate dead, counterspell, dispel magic, fireball
4th level (3 slots): blight, dimension door
5th level (3 slots): cloudkill, scrying
6th level (1 slot): disintegrate, globe of invulnerability
7th level (1 slot): finger of death, plane shift
8th level (1 slot): dominate monster, power word stun
9th level (1 slot): power word kill

The lich has advantage on saving throws against any effect that turns undead.

3 legendary actions; one at a time at the end of another creature’s turn, regaining spent uses at the start of its turn

3/day. If the lich fails a saving throw, it can choose to succeed instead.

Melee Spell Attack: +12 to hit, reach 5 ft.. Hit: 15 (3d6 + 5) cold damage.
The target must succeed on a DC 18 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 lich casts a cantrip.

The lich uses its Paralyzing Touch.

Costs 2 legendary actions. The lich fixes its gaze on 1 creature it can see within 10 ft. of it. The target must succeed on a DC 18 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 for it, the target is immune to the lich's Frightening Gaze for the next 24 hours.

Costs 3 legendary actions. Each non-undead creature within 20 feet of the lich must make a DC 18 Constitution saving throw against this magic, taking 21 (6d6) necrotic damage on a failed save, or half as much damage on a successful one.