Blind Song. All hostile creatures that can hear the Hydra sing must make a DC15 Wisdom saving throw at the start of their turn. If the Hydra sings with more than one head, all hostile creatures must subtract 1d4 from their saving throw. On a failure, the creature forgets the Hydra is there.

Multiattack. The Hydra can use a claw attack and a bite attack on its turn.

Bite. Melee Weapon Attack: +5 to hit, reach 15 ft., one target. Hit: 9 (1d10+3) piercing damage. 

Claws. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 8 (1d8+3) slashing damage. If the Hydra succeeds on a claw attack, it can choose to grapple the target instead of dealing damage. (The hydra can only grapple one creature at a time and cannot use its claw attack while grappling a creature.)

Sing. The Hydra can use a bonus action to stop singing or to start its song with an additional head.

Regrowth. The Hydra can use a bonus action to begin regrowing a head that has died. It takes three Regrowth bonus actions to fully regrow a head. It regains 20 hit points each time it uses Regrowth.

Multireact. The Hydra can make two reactions per round as long as it has three or more heads.

Wail. After taking damage, the Hydra can use a reaction to wail. Creatures within 5ft of the Hydra when it wails take 2d4 psychic damage.