Wastrilith Lich

Breathless: The Wastrilith Lich dose not need to breath.

Corrupt Water. At the start of each of the Wastrilith Lich’s turns, exposed water within 30 feet of it is befouled. Underwater, this effect lightly obscures the area until a current clears it away. Water in containers remains corrupted until it evaporates.

A creature that consumes this foul water or swims in it must make a DC 18 Constitution saving throw. On a successful save, the creature is immune to the foul water for 24 hours. On a failed save, the creature takes 14 (4d6) poison damage and is poisoned for 1 minute. At the end of this time, the poisoned creature must repeat the saving throw. On a failure, the creature takes 18 (4d8) poison damage and is poisoned until it finishes a long rest.

If another demon drinks the foul water as an action, it gains 11 (2d10) temporary hit points.

Magic Resistance. The Wastrilith Lich has advantage on saving throws against spells and other magical effects.

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

Undead Fortitude. If damage reduces the wastrilith lich to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the Wastrilith Lich drops to 1 hit point instead.

Undertow. As a bonus action when the Wastrilith Lich is underwater, it can cause all water within 60 feet of it to be difficult terrain for other creatures until the start of its next turn.

Spellcasting. The Wastrilith Lich is an 18th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). The Wastrilith Lich has the following spells prepared:

Cantrips (at will) : Chill Touch, Mind Sliver

1st level (4 slots) : Bane, Chaos Bolt, Hellish Rebuke, Inflict Wounds

2nd level (3 slots) : Darkness, Hold Person, Heat Metal

3rd level (3 slots) : Animate Dead, Bestow Curse, Counter Spell

4th level (3 slots) : Blight, Control Water, Summon Greater Demon

5th level (3 slots) : Cloud Kill, Contagion, Dominate Person

6th level (1slot) : Disintegrate

7th level (1slot) : Finger of Death

8th level (1slot) : Tsunami

9th level (1slot) : Power Word Kill

Multiattack. The Wastrilith Lich uses Grasping Spout and makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 30 (4d12 + 4) piercing damage.

Claws. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 18 (4d6 + 4) slashing damage.

Grasping Spout. The Wastrilith Lich magically launches a spout of water at one creature it can see within 60 feet of it. The target must make a DC 17 Strength saving throw, and it has disadvantage if it’s underwater. On a failed save, it takes 22 (4d8 + 4) acid damage and is pulled up to 60 feet toward the Wastrilith Lich. On a successful save, it takes half as much damage and isn’t pulled.