Chuulian Hydra

Amphibious. The hydra can breathe air and water.

Magic Resistance. The hydra has advantage on saving throws against magical effects.

Magic Sense. The hydra senses magic within 120 ft of it at will. This trait otherwise works as the detect magic but isn't itself magical. 

Multiple Heads. The hydra has three 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 50 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 or necrotic damage since its last turn. The hydra regains 20 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.

Aura of Sickness. The noxious breathe of the hydra creates an aura of poison that seeps into the pores of creatures around it. When a creature starts its turn within 30 ft of the hydra, it must succeed a DC 17 Constitution saving throw or take 9 (2d10) poison damage and become poisoned. While poisoned in this way, a creature cannot regain hit points. A creature can repeat the saving throw at the end of each other their turns, ending the effect on a success. Creatures that succeed the saving throw are immune to this effect for 24 hours. 

Multiattack. The hydra makes as many bite attacks as it has heads. It can replace two  of these attacks with a Tail Slam attack. 

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 11 (1d12 + 5) piercing damage. If the creature is a Large or smaller then it is grappled (escape DC 17). While the hydra has a creature grappled in its mandibles, that head cannot make attacks on other creatures. The grappled creature must succeed on a DC 17 Strength saving throw or take 16 (3d10) bludgeoning damage as tentacles flood forth and begin to crush them. 

Tail Slam. The large lobster-like tail whips toward the hydra's foes. Creatures in a 15 foot cone struck by the tail must make a DC 17 Dexterity saving throw or take 18  (3d8+5) bludgeoning damage and be knocked prone on a failed save. On a success, the target takes half damage and is not knocked prone. 

Interrupt Arcana (1/ short rest). The hydra uses its action to emit an arcane pulse, severing creature's connections to enchanted items within 45 ft. If a creature within the area is attuned to one or more magic items, they must succeed on a  DC 12 Intelligence saving throw or instantly be de-attuned from these items. 

Probing Doubt. When the hydra is the target of an attack by a creature it can see within 60 ft, the hydra can use its reaction to telepathically fill that creature's mind with images of power and desire that the hydra could grant it. This momentary wave of incited doubt imposes disadvantage on the attack roll. 

The hydra can take 2 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. 

Tail Slam.  The hydra makes a tail slam attack.

Squat Defence. The hydra tenses, closing the gaps in its plated armour. Its AC increases by 5 until the start of its next turn. 

Summon Aquatic Allies (Costs 2 actions).  The hydra screeches to the ocean, small rifts appearing pouring sea water across the ground as 4 (1d4 + 2) giant crab are summoned. The crabs roll an initiative as a group and act in accordance with the hydra's will.