Echolocation. The bat can't use its blindsight while deafened.
Keen Hearing. The bat has advantage on Wisdom (Perception) checks that rely on hearing.
Death Bite. The bat can, on an attack that hits, add 2d8 necromantic damage to the attack. After doing this, the bat must wait a round before attacking again.
Bite. Melee Weapon Attack: d20+5 to hit, reach 5 ft., one creature. Hit: d4+3 piercing damage.