Uumlaut

Keen Smell. The swarm has advantage on Wisdom (Perception) checks that rely on smell.

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 rat. The swarm can't regain hit points or gain temporary hit points.

Light Sensitivity. Whenever the uumlaut is exposed to bright light from a source other than sunlight, it must succeed on a DC 15 Constitution saving throw or become stunned until the end of its next turn.

Pack Tactics. The gremlin has advantage on an attack roll against a creature if at least one of the uumlaut allies within 5 feet of the creature and the ally isn’t incapacitated.

Rattle Spawn. For every 6 seconds (1 round) the uumlaut goes without its rattle, the uumlaut spawns 1d6 + 1 new gremlins if it has at least half its hit points. Each new gremlin has hit points equal to the original uumlaut's hit point total.

Bite. Melee Weapon Attack: +2 to hit, reach 0 ft., one target in the swarm's space. Hit: 7 (2d6) piercing damage, or 3 (1d6) piercing damage if the swarm has half of its hit points or fewer.

Claws. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d4 + 2) slashing damage.