Astral 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 75 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 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.

Gargantuan Stature. A creature adjacent to the Astral Hydra may spend half its movement to attempt a DC 21 Strength (Athletics) check. On a success, the creature climbs onto the back of the Astral Hydra. While on the Astral Hydra's back, a creature has advantage on melee attacks against it.

Intense Gravitational Pull. All movement away from the Astral Hydra is halved. Any time a creature moves more than 30 ft. in a turn, that creature requires a DC 15 Strength saving throw or be pulled 30 ft. towards the Astral Hydra.

Meteor Strike. The first time the Astral Hydra starts a turn with two heads or less, the Astral Hydra can spit a Meteor Swarm shot per head remaining (Dexterity saving throw DC 22).

Escape from the Astral Calamity. If the Astral Hydra dies, the black hole at the center of its mass begins to implode. Each creature within 300 ft. must succeed a DC 15 Strength saving throw, within 150 ft. a DC 23, within 50 ft. a DC 30, or be sucked into the black hole and utterly destroyed. Afterwards the black hole explodes. Each creature within 300 ft. must succeed a DC 25 Dexterity saving throw or take 150 (30d10) force damage, half as much on a successful save.

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

Bite. Melee Weapon Attack: +10 to hit, reach 20 ft., one target. Hit: 27 (5d10 + 5) piercing damage. A creature bitten by the Astral Hydra is grappled (escape DC 21).

Swallow. The Astral Hydra makes one bite attack against a large or smaller creature it is grappling. If the attack hits, that creature takes the bite's damage and is swallowed.
While swallowed, the creature is Blinded and continues to be Grappled (escape DC 21). While within the Astral Hydra, attacks bypass all the hydra's immunities and resistances, the creature has total cover against all attacks and spells not originating from within the hydra, and the creature takes 14 (4d6) acid damage at the start of each of the Astral Hydra's turns. At the start of a creatures 4th turn within the Astral Hydra reaches its stomach, the black hole, and is utterly destroyed. If the Astral Hydra loses that particular head, the creature is freed.

The Astral Hydra can take 3 legendary actions that can be used at a time and only at the end of another creature's turn. The Astral Hydra regains spent legendary actions at the start of his turn.

Astral Hydra's Gaze (Costs 1 Action). A creature within 60 ft. of a hydra head must succeed a DC 20 Charisma saving throw or be frightened, repeating the saving throw at the beginning of each of its turns to recover.

Reverse Breath Weapon: Gravity Well (Costs 2 Action). Creatures within a 60 ft. cone of the Astral Hydra's head must succeed on a Strength saving throw DC 20 or be pulled 40 ft. towards the hydra's head, the Astral Hydra can make an attack on each creature who failed if they are within it's Bite range. If a creatures succeeds, they are only pulled 10 ft. towards the hydra.