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

Turn Immunity. The bat is immune to effects that turn undead.

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

Undead Fortitude. If damage reduces the bat to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the bat drops to 1 hit point instead.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 9 (2d6 + 3) piercing damage and 7 (1d4+3) acid damage.

Acid vomit  (Recharge 5–6). The bat vomits acid in a 15 ft cone. Every creature in that area must make a dexterity saving throw with a DC of 13. On a failed save they take 12 (3d8) acid damage or half as much damage on a successful one.