Wail. When a creature that hits the Hydra with a melee attack is within 5ft. of the creature it takes 3d4 psychic damage.

Grapple. If the Hydra succeeds on a bite attack it can chose to grapple the creature instead of dealing damage if it is a size equal to or smaller than it.

Multiple Heads. The Hydra has seven heads. While it has more than one head, the Hydra has advantage on saving throws against being blinded, deafened, frightened, stunned, and knocked unconscious. 
If the Hydra takes 35 or more damage in a single turn, one of its heads dies. If all of its heads die, the Hydra dies.

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

Song. If a creature is hostile to the Hydra when it begins its song the creature make a Wisdom saving throw DC 16. On a failure the creature drops out of combat and forgets that the Hydra is there, on a success the creature has disadvantage on all attacks against the hydra and must repeat the saving throw at the end of each of its turns if the song has not stopped.

Multiattack. The hydra can make three attacks with any combination of the Bite or Claw attacks

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., 1 target. Hit: 13 (3d6 + 4) piercing damage. 

Claw. Melee Weapon Attack: +8 to hit, reach 10 ft., 1 target. Hit: 12 (2d8 + 4) slashing damage.

Sing. The Hydra can end or start its song as a bonus action

Regrowth. The Hydra can use a bonus action to regrow two of its heads. It regains 60 hit points. The Hydra can't use this bonus action if it has taken more than 10 fire damage since the end of its last turn.

Shriek. As a reaction to being hit by a melee attack, the Hydra sings its song, granting it a +5 to its AC against the attack.