Wastrilith

Armor Class 18 (natural armor)
Hit Points 157 (15d10 + 75)
Speed 30 ft., swim 80 ft.Saving Throws Str +9, Con +10
Damage Resistances cold, fire, lightning; bludgeoning, piercing, and slashing from nonmagical attacks
Damage Immunities poison
Condition Immunities poisoned
Senses darkvision 120 ft., passive Perception 11
Languages Abyssal, telepathy 120 ft.
Challenge 13 (10000 XP)     Proficiency Bonus +5