Celestial Eel (HB)

Echolocation. The creature can’t use its blindsight while deafened

Fire Form. The celestial eel can move through a space as narrow as 1 inch wide without squeezing. A creature that touches the monster or hits it with a melee attack while within 5 ft of it takes 2 (1d4) fire damage. In addition, the monster can enter a hostile creature’s space and stop there. The first time it enters the creature’s space on a turn, that creature takes 2 (1d4) fire damage and catches fire; until someone takes an action to douse the fire, the creature takes 2 (1d4) fire damage at the start of each of its turns. 

Divine Fire. The Celestial Eel is made of Divine Fire which is not extinguished by water

Spell Immunity. the celestial eel can choose to be unaffected a spell that affects it up to two times per day.

Flame Grasp. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 6 (1d6 + 3) fire damage. 

Divine Flame. RangedWeapon Attack: +4 to hit, range 50 ft., 1 target. Hit: 4 (1d6 + 2) Radiant damage.