Cats of Ulthar

Feline Terror. Each creature in the swarm must succeed on a DC 12 Wisdom saving throw or fall prone and become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the swarm's Feline Terror for the next 24 hours.

Keen Senses. The swarm has advantage on Wisdom (Perception) checks that rely on sight, hearing, or smell.

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 hp or gain temporary hp.

Bites. Melee Weapon Attack: +6 to hit, reach 0 ft., up to two creatures in the swarm's space. Hit: 14 (4d6) piercing damage, or 7 (2d6) piercing damage if the swarm has half of its hit points or fewer.