Brown Scaver

Blood Frenzy. The Scaver has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Void Breathing. The Scaver does not need to breath air or water

Bite. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 22 (3d10 + 8) piercing damage plus (3d5 Poison) and if they crit they swallow the creature once inside they take 5d10 acid damage on the players turn. The player when inside has a +5 bonus to hit and all hits are crits the scaver can vomit the player up if they want

Gas cloud (recharge 4-6). The scaver exhales  poisonous gas in a 60-foot cone. Each creature in that area must make a DC 16 Constitution saving throw, taking  42 (12d6) poison damage on a failed save, or half as much damage on a successful one. It stays there for 2 turns