Lantern Hagfish

The lantern hagfish is resistant to the charmed, frightened, paralyzed, and stunned conditions, and it has advantage on saves against spells or effects that would alter its form.

The lantern hagfish sheds bright light in a 5- to 20-foot radius and dim light for an additional number of feet equal to the chosen radius. The hagfish can alter the radius as a bonus action. In addition, the hagfish has advantage on attack rolls against a creature if at least one other hagfish is within 5 feet of the creature and that other hagfish isn't incapacitated.

The lantern hagfish doesn't require air or sleep, and it is immune to the effects of Void terrain. In addition, it can swim through the Void as if through water.

Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 3 (1d4+1) piercing damage. If the target is a prone creature that isn't a Construct or Undead, the hagfish can burrow into the creature's flesh. While burrowed into a creature, the hagfish can't attack, and at the start of each of the hagfish's turns, the target loses 3 (1d4+1) HP as the hagfish consumes it from the inside. The burrowed hagfish moves with the target whenever the target moves, requiring none of the hagfish's movement. It can detach itself by spending 5 feet of its movement on its turn. It does so after it consumes 6 HP worth of flesh from the target or the target dies. A creature, including the target, can take its action to extract the burrowed hagfish by succeeding on a DC 11 WIS (Medicine) check.

While on a solid surface, the lantern hagfish slides up to 15 feet in a straight line and can move through the space of any creature without provoking opportunity attacks. Each space on that surface along that line is coated in slime until the start of the hagfish's next turn. Each creature in that line, and any creature that enters a slime-coated space for the first time on a turn must succeed on a DC 11 DEX save or fall prone.