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 creature. The swarm can't regain Hit Points or gain Temporary Hit Points.
Swarm of Grasping Hands. Melee Attack Roll: +4, reach 5 ft. Hit: 20 (4d8 + 2) Necrotic damage, or 11 (2d8 + 2) Necrotic damage if the swarm is Bloodied. If the target is a Medium or smaller creature, it has the Prone condition.