Ultroloth

If the ultroloth dies outside Gehenna, its body dissolves into ichor, and it gains a new body instantly, reviving with all its Hit Points somewhere in Gehenna.

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

The ultroloth casts one of the following spells, requiring no Material components and using Intelligence as the spellcasting ability (spell save DC 17):

At will: Alter Self, Clairvoyance, Detect Magic

1/day each: Dimension Door, Fireball (level 5 version), Wall of Fire

The ultroloth casts Dispel Magic, Invisibility (self only), Misty Step, or Suggestion, requiring no Material components and using the same spellcasting ability as Spellcasting.

The ultroloth uses Hypnotic Gaze and makes two Mercurial Whip attacks.

Melee Attack Roll: +9, reach 15 ft. Hit: 25 (6d6 + 4) Force damage, and the ultroloth can teleport the target up to 10 feet to an unoccupied space the ultroloth can see that isn't in the air.

Wisdom Saving Throw: DC 17, each creature in a 30-foot Cone. Failure: 10 (3d6) Psychic damage, and the target has the Stunned condition until the start of the ultroloth's next turn. Success: The target is immune to this ultroloth's Hypnotic Gaze for 24 hours.