Astral Portal Hydra

Hold Breath. The hydra can hold its breath for 8 hours.

One with portals. The hydra can open a number of portals equal to its number of heads it can pass a head through the portal for an extra 2d8 force damage and endless range with its bite attacks. it also has advantage on bite attacks made through portals unless the target passes a dc 25 perception check. if it makes it breath attack through a portal the portal is considered the point of origin for the attack and does an extra 3d8 damage with an increased dc 25 save.

Astral Healing. the hydra regains hp equal to its number of heads at the start of its turn.

Multiple Heads. The hydra has as many heads as the Mk2 had before it died minimum of 15. 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, two 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. The hydra regains 15 hit points for each head regrown in this way.

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

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction

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

Cold hearted. The hydra cannot be seduced by a player.

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

Bite. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 25 (4d10 + 5) piercing damage. (if through a portal add 10 (2d8) force damage) if made not through a portal the hydra can attempt to grapple. while grappled in this way the hydra can use constrict and all bite attacks hit that target.

Constrict. the hydra coils a neck around the target and constricts them for an amount equal to xd6 damage. x being 1/4 of the grappled creatures hit die rounded down. while grapped in this way the players cannot use any form of teleportation or dimensional travel. doing so results in an attack of opportunity for the hydra to use constrict again

Breath attack. (Recharge 4-6)  Single Target The hydra unleashes attacks of its foul astral breath causing 20 (4d8) force damage on a failed same and half as much on a success DC 20

Teleport: The Hydra magically teleports, along with any equipment it is wearing or carrying, to an unoccupied space.

Astral Drain. The hydra drains the life force from nearby creatures using wisps of astral energy. each creature within 100 feet of the hydra makes a dc 20 constitution saving throw. on a failure roll a 1d6 on a success half the damage taken

1.  Take 1d8 damage
2.  Take 2d8 damage and have its speed halved until the end of the hydras next turn.
3. Take 3d8 damage and the hydra heals half as much
4. Take 3d8 damage and the hydra heals the same amount.
5. Take 3d8 damage and the hydra heals twice as much
6. The hydra regains hp equal to the amount it deals next turn

The Portal Hydra Mk2 can take 3 legendary actions. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Portal  monster regains spent legendary actions at the start of its turn.

Breath attack. Multi Target (1d4) The hydra unleashes attacks of its foul astral breath causing 4d8 force damage on a failed same and half as much on a success DC 20

Teleport the Hydra uses its teleport action 3 times. making an attack after each teleport

Astral Shred (3 Actions) The portal hydra uses astral shred.

Astral Drain. The hydra Uses Astral Drain with a roll of 6 on the d6 now being that it instead gains immunity to damage until its next turn ends