Echolocation. The bat can't use its blindsight while deafened.

Keen Hearing. The bat has advantage on Wisdom (Perception) checks that rely on hearing.

Multiattack. The bat makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 7 (1d8 + 3) piercing damage.

Claw. Melee Weapon Attack: + 5 to hit, reach 5 ft., one creature. Hit:  6 (1d6 + 3) slashing damage.

Flash (1/day). At the end of a players turn, the dirge bat can change its spot in the initiative order to initiative count 21.

Mutate. If the bat mutates, target a hostile creature within 60 ft. must make a DC 12 Constitution saving throw, taking 22 (4d10) necrotic damage on a failed save, or half as much damage on a successful one.