Cocytalisk (5e Creature)
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Large fiend, any evil alignment
Saving Throws Dex +9, Wis +6
Ice Walk. The cocytalisk can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra movement.
Innate Spellcasting. The cocytalisk's innate spellcasting ability is Wisdom (spell save DC 14). The cocytalisk can innately cast the following spells, requiring no material components, with the exception of simulacrum which requires all material components other than the powdered ruby:
Multiattack. The cocytalisk makes three attacks: one with its bite and two with its claws.
Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 19 (3d8 + 6) piercing damage.
Claw. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 15 (2d8 + 6) slashing damage.
Frozen Aura (Recharge 5-6). The cocytalisk creates an area of freezing cold in a 30-foot radius around itself. Each creature in that area must make a DC 15 Constitution saving throw, taking 10 (3d6) cold damage on a failed save, or half as much damage on a successful one. If a creature that fails its saving throw is touching the ground, it becomes frozen to the ground and is restrained. A creature restrained by the ice can use its action to make a DC 15 Strength check, freeing itself on a success.
Icy Sprint. If the cocytalisk started its turn on icy terrain, it can use this action to move up to half its movement speed.
Frozen Armor. Upon being targeted by a bludgeoning, slashing, or piercing attack, the cocytalisk can reduce the incoming damage by 30, but its AC becomes 10 under the end of its next turn.
These frozen fiends appear as tall monstrous horned skeletons on first glance, but despite their emaciated appearance they have skin and muscle... and are actually both quite agile and very strong. They also have limited innate spellcasting ability and are far more intelligent than they appear. A cocytalisk will often travel with a simulacrum of itself, either together to serve as a decoy, or isolated to act as a lure for would-be demon slayers. But, a cocytalisk is not above making a simulacrum of someone else for its own purposes.