Ancient Emerald Hydra

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 Psychic damage since its last turn. The hydra regains 10 hit points for each head regrown in this way.

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.

Interference Aura. Enemies within 30 feet must make a DC 22 Intelligence saving throw every round to maintain spells that require concentration.

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

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 (1d12 + 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 dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) Check.

Psionics. The dragon 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 24 Intelligence saving throw or take 14 (2d6 + 7) force damage and be knocked prone. The Hydra then can move up to half its movement speed.

Psionic Decimate (Costs 3 Actions).  The Hydra unleashes its latent telekinetic power to wreck havoc on all creatures within 30 feet. All creatures in this range must make a DC 24 Intelligence saving throw or be pulled toward the Hydra and be knocked prone taking (2d6 + 7) force damage and be knocked prone. The Hydra then can make 1 attack action.