Circle of Vermin Subclass
Spider Climb.The swarm can climb difficult surfaces, including upside down on 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 vermin. The swarm can't regain hit points or gain temporary hit points outside of specific Circle of Vermin features.
Bites. Melee Weapon Attack: your spell attack modifier to hit, reach 5ft., one target. Hit (1d6 + PB) piercing damage. Infestation.If a swarm is sharing a creatures space it can attempt to infest the creature. A creature must succeed on a Constitution saving throw against your spell save DC or become infested. While infested, the creature has disadvantage on attack rolls, and the Swarm Companion cannot move or attack while having total cover against attacks and other effects outside of the infested creature. On subsequent turns, you must use your bonus action to command the Swarm Companion to maintain infestation over the target, or the infestation ends. Also, at the end of each of its turns, the creature can make a Constitution saving throw, ending the infestation on a success. If a targets saving throw is successful or the effect ends for it, the target is immune to Infestation for the next 24 hours.