Lich

Medium undead, any evil alignment

Armor Class 17 (natural armor)
Hit Points 135 (18d8 + 54)
Speed 30 ft.

Saving Throws Con +10, Int +12, Wis +9
Skills Arcana +18, History +12, Insight +9, Perception +9
Damage Resistances cold, lightning, necrotic
Damage Immunities poison; bludgeoning, piercing, and slashing from nonmagical attacks
Condition Immunities charmed, exhaustion, frightened, paralyzed, poisoned
Senses truesight 120 ft., passive Perception 19
Languages Common plus up to five other languages
Challenge 21 (33,000 XP)

Legendary Resistance (3/Day). If the lich fails a saving throw, it can choose to succeed instead.
Rejuvenation. 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.
Spellcasting. 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): acid arrow, detect thoughts, invisibility, 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
Turn Resistance. The lich has advantage on saving throws against any effect that turns undead.

Actions

Paralyzing Touch. Melee Spell Attack: +12 to hit, reach 5 ft., one creature. Hit: 10 (3d6) 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.

Legendary Actions

The lich 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 lich regains spent legendary actions at the start of its turn.
Cantrip. The lich casts a cantrip.
Paralyzing Touch (Costs 2 Actions). The lich uses its Paralyzing Touch.
Frightening Gaze (Costs 2 Actions). The lich fixes its gaze on one creature it can see within 10 feet 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 gaze for the next 24 hours.
Disrupt Life (Costs 3 Actions). Each living 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.