Spider Climb. If the swarm has a Climb Speed, the swarm can climb difficult surfaces, including along ceilings, without needing to make an ability check.
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 insect. The swarm can't regain Hit Points or gain Temporary Hit Points.
Actions
Bites. Melee Attack Roll: +3, reach 5 ft. Hit: 6 (2d4 + 1) Poison damage, or 3 (1d4 + 1) Poison 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.