Hold Breath. The hydra can hold its breath for 1 hour.

Multiple Heads. The hydra has five heads. While it has more than one head, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Whenever the hydra takes 25 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra dies.

At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it has taken fire damage since its last turn. The hydra regains 10 hit points for each head regrown in this way.

As the hyrda loses heads and it begins to get more and more desperate it will begin to rely on its psionic abilities. 

Psionics

CHARGES: 15 • RECHARGE: 1d6 • FRACTURE: 18

Feedback Aura. Each time the Hydra takes damage, all creatures within 30 feet must succeed on a DC 15 Intelligence saving throw or else take 7 (2d6) psychic damage.

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Wakeful. While the hydra sleeps, at least one of its heads is awake.

AMPLIFY

Casting Time: 1 action Range: Self  Duration: 1 minute Charges: 4

The Hydra focuses the power of its mind and wreaths its teeth, claws, and tail in glowing psionic force. For the next minute, all of its melee attacks deal an extra 3d8 psychic damage.

FLAY

Casting Time: 1 bonus action Range: Self (60-foot cone) Duration: 1 round Charges: 2 per d6 of damage

As a bonus action, the Hydra blasts pure psionic energy from its eyes, frying the brains of all creatures in a 60-foot cone. Each creature in that area must make a DC15 Intelligence saving throw. On a failed save, it takes psychic damage of a d6 per 2 charges spent, or half as much damage on a successful one.

Multiattack. The hydra makes as many bite attacks as it has heads.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage.

The Hydra can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The Hydra regains spent legendary actions at the start of its turn.

Detect. The Hydra makes a Wisdom (Perception) Check

Psionics. The Hydra uses a psionic ability.

Psionic Shift (Costs 2 Actions). The Hydra releases a wave of telekinetic energy from its mind. Every creature within 15 feet must make a DC 15 Intelligence saving throw or take 12 (2d6 + 5) force damage and be knocked prone. The dragon then can move up to half its movement speed.