Choas 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 fire 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.

Different Mutated Heads - The Choas Hydra starts with 1 normal demon head, and 1 Central head. From there roll a d8 to determine what the rest of the heads are. Whenever a head is split in two, roll for the new heads.

1. Rabid central head: This central head attacks the other heads that are out of range of healing, cutting them off to grow more heads. Once per turn the hydra may automatically replace one of its heads with two others. It uses this ability to adapt to the enemies it is fighting. If the central head is the only one left it will attempt to fight the players itself
2. Noozle Head: Instead of attacking this head spews poison. All creatures in a 15ft cone must make a DC 12 CON save or take 3d6 acid damage and become poisoned. Successful save's take half damage and are not poisoned
3. Monocular head: This head's eyes glow with a terrifying brightness. It has an anti-magic cone of 60 feet. 
4. Mosquito Head: Damage dealt by this head heals the hydra, furthermore, when this head is cut off all creatures in a 10ft sphere centered on the head must make a DC 12 DEX save or take 3d6 piercing damage from a swarm of mosquito's that fly out of the corrupted stump
5. Tentacle Tongue Head This head has a reach of 20ft. Creatures hit by attacks from this head are pulled 10 feet towards the hydra. If they would be pulled into the hydra they must make a DC 15 STR save or become grappled
6. Flesh Stump head: Where a head should have grown, there is only a meaty stump. This heads attacks deal bludgeoning damage and targets hit by these heads must make a DC 15 STR Save or be knocked prone
7. Warp-Tooth Head: This Head's teeth are formed of warp stone, on a hit the target must succeed on a DC 12 WIS Save or suffer the effects of the confusion spell
8. Abyss touched head:  Eldritch Burst. Great bolts of eldritch energy are arcing out
   of this head, having long since burnt away its mouth. Four line's that are 60 feet long and 5 feet wide blasts out in random directions (Assign numbers to the lines on a combat grid and roll a dice) Any creature in the line must make a DC 15 Dexterity saving throw, a creature takes 7 (2d6) lightning damage and 7 (2d6)
   force damage on a failed save, or half as much damage on
   a successful one.

Multiattack. The hydra makes as many bite attacks as it has heads (that are not the central head). Its different heads can make different attacks

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage and 3 (1d6) poison damage