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 bat. The swarm can't regain Hit Points or gain Temporary Hit Points.
Actions
Bites. Melee Attack Roll: +4, reach 5 ft. Hit: 5 (2d4) Piercing damage, or 2 (1d4) Piercing damage if the swarm is Bloodied.
RollQuest ships the D&D 2024 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.