Cinder Swarm

Illumination. The swarm sheds bright light in a 30-foot radius and dim light in an additional 30 feet.

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny elemental. The swarm can't regain hit points or gain temporary hit points.

Water Susceptibility. For every 5 feet the swarm moves in water, or for every gallon of water splashed on it, it takes 1 cold damage.

Multiattack. The swarm makes one attack against each creature whose space it occupies. 

Burning Bites. Melee Weapon Attack: +10 to hit, reach 0 ft., one target in the swarm's space. Hit: 14 (4d6) piercing damage plus 14 (4d6) fire damage, or 7 (2d6) piercing damage plus 7 (2d6) fire damage if the swarm has half of its hit points or fewer. If the target is a creature, it must succeed on a DC 15 Dexterity saving throw or catch fire; until someone takes douse the fire, the creature takes 5 (1d10) fire damage at the start of each of its turns.