Swarm of Bloodbloaters (5e Creature)
From D&D Wiki
Medium swarm of Tiny oozes, unaligned
Skills Stealth +3
Amorphous. The bloodbloater can move through a space as narrow as 1 inch wide without squeezing.
Spider Climb. The bloodbloater 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 bloodbloater. The swarm can't regain hit points or gain temporary hit points, except through the Bites action.
Bites. Melee Weapon Attack: +3 to hit, reach 5 ft., one target in the swarm's space. Hit: 7 (2d6) necrotic damage, or 5 (2d4) necrotic damage if the swarm has half of its hit points or fewer, and the swarm regains an equal number of hit points if the target is not a construct, undead, elemental, or object. If this would incur a Constitution saving throw to maintain concentration, the DC is 15 instead of 10.
In swarms of often over 600, bloodbloaters can easily drain lesser creatures in seconds.