Bat King

Echolocation. Cannot use blindsight while deafened.

Keen Hearing. The Bat king has advantage on wisdom (perception) checks that rely on hearing.

Sound Sensitivity. Whenever he takes 11 or more thunder damage from a single attack, he must make a constitution save with a DC of 5 + the damage taken or become deafened for 1 turn.

Hail to the King. The bat king can summon a swarm of bats once between rests.

Blood Thirst. The bat king can regain HP from a creature by sucking its blood. Targets of his bite attack must make a constitution save or the bat king regain the amount of HP the target lost in the attack.

Multi-attack. The bat king makes two attacks, only one of which may be a bite.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (2d4 + 2) piercing damage. Target must make a constitution save of DC 12 or the bat king's HP increases by their damage.

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

Flight. If the bat king is successfully hit with a melee attack, as a reaction he can grab on to his attacker and take flight. Attacker must make a dexterity save greater than the king's strength roll to escape being lifted into the air.

If the bat king is possessed he gains double HP and is able to perform one of the possessing demon's spells.