Homebrew
Keen Smell. The swarm has advantage on Wisdom (Perception) checks that rely on smell.
Magic Resistance. The swarm has advantage on saving throws against spells & other magical effects.
Pack Tactics. The swarm has advantage on an attack roll against a creature if at least one of the swarm's allies is within 5' of the creature & the ally isn't incapacitated.
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 cat. The swarm can't regain hit points or gain temporary hit points.
Claws. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 29 (6d8 + 2) slashing damage, or (3d8 + 2) slashing damage if the swarm has half of its hit points or fewer.
Bite. Melee Weapon Attack: +1 to hit, reach 5 ft., one target. Hit: 8 (2d6 + 1) piercing damage, or 4 (1d6 + 1) piercing damage if the swarm has half of its hit points or fewer.
Any climate, any terrain