Flee, Mortals! (5e)
Multitudinous. The swarm has advantage on opportunity attacks, and creatures provoke opportunity attacks from the swarm even if they take the Disengage action before leaving the swarm's reach.
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.
Bite. Melee Weapon Attack: +4 to hit, reach 0 ft., one target in the swarm's space. Hit: 10 (4d4) piercing damage, or 5 (2d4) piercing damage if the swarm has half their hit points or fewer.
Distracting Buzz. If the swarm has at least half their hit points remaining, they emit an annoying drone, buzz, or skittering sound. Until the start of the swarm's next turn, creatures within 15 feet of the swarm who can hear them have disadvantage on attack rolls against creatures other than the swarm.
Desert, Forest, Grassland, Hill, Swamp, Underdark, Urban