Sources
CR
Type
Size
Alignment
Movement

Renwick

Medium Undead, Lawful Evil

Armor Class 17 (natural armor)
Hit Points 135 (18d8 + 54)
Speed 30 ft.
STR
11
+0
DEX
16
+3
CON
16
+3
INT
20
+5
WIS
14
+2
CHA
16
+3
Saving Throws CON +10, INT +12, WIS +9
Skills Arcana +19, History +12, Insight +9, Perception +9
Damage Resistances cold, lightning, necrotic
Damage Immunities poison; bludgeoning, piercing, 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)

Traits

Legendary Resistance (3/Day). If Renwick 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.

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

Spellcasting

Spellcasting. Renwick is an 18th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). Renwick 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, Melf's 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): time stop

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

Cantrip. Renwick casts a cantrip.

Paralyzing Touch (Costs 2 Actions). Renwick uses its Paralyzing Touch.

Frightening Gaze (Costs 2 Actions). Renwick 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 non-undead creature within 20 feet of Renwick 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.