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

Slam. Melee Weapon Attack: +4 to hit, reach 0 ft., one creature in the swarm's space. Hit:5 (2d4) bludgeoning damage, or 2 (1d4) bludgeoning damage if the swarm has half of its hit points or fewer.