Ice Devil

If the Ice Devil dies outside the Nine Hells, its body disappears in sulfurous smoke, and it gains a new body instantly, reviving with all its Hit Points somewhere in the Nine Hells.

The Ice Devil has Advantage on saving throws against spells and other magical effects.

The Ice Devil makes three Ice Spear attacks. It can replace one attack with a Tail attack.

Melee or Ranged Weapon Attack: +10 to hit, reach 5 ft. or range 30/120 ft., one target. Hit: 11 (2d8 + 5) Piercing damage plus 10 (3d6) Cold damage. Until the end of its next turn, the target can't take a Bonus Action or Reaction, its Speed decreases by 10 feet, and it can move or take one action on its turn, not both. Hit or Miss: The spear magically returns to the Ice Devil's hand immediately after a ranged attack.

Melee Attack: +2 to hit, reach 10 ft., one target. Hit: 18 (4d8) Cold damage.

The Ice Devil casts Wall of Ice (level 8 version), requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 17).