Echolocation. The swarm can't use its blindsight while deafened. Keen Hearing. The swarm has advantage on Wisdom (Perception) checks that rely on hearing. 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 bat.
Blooddrinker Bites (swarm has more than half HP). Melee Weapon Attack: +4 to hit, reach 0 ft, one creature in the swarm's space. Hit: (2d4) piercing damage plusĀ (1d4) necrotic damage, the swarm heals by half the amount of necrotic damage dealt, rounded down. Targets hit with this attack must also succeed on a Constitution saving throw (DC 13) or contract Sanguinare Vampiris. Blooddrinker Bites (swarm has half HP or less). Melee Weapon Attack: +4 to hit, reach 0 ft, one creature in the swarm's space. Hit: (1d4) piercing damage plus (1d4) necrotic damage, the swarm heals by half the amount of necrotic damage dealt, rounded down. Targets hit with this attack must also succeed on a Constitution saving throw (DC 13) or contract Sanguinare Vampiris.