Crystal Hydra

Hold Breath. The hydra can hold its breath for 1 hour.

Immutable Form. The Crystal Hydra is immune to any spell or effect that would alter its form.

Multiple Heads. The Crystal 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 Crystal 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 Thunder damage since its last turn. The Crystal Hydra regains 15 hit points for each head regrown in this way.

Reactive Heads. For each head the Crystal Hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

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

Multiattack. The Crystal Hydra makes as many bite attacks as it has heads.

Bite. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 10 (2d6 + 5) piercing damage plus 2 (1d4) radiant damage.

Breath Weapon (Recharge 5–6). The Crystal Hydra exhales Eight multicolored rays of light. Each ray is a different color and has a different power and purpose. Each creature in a 60-foot cone must make a DC 17 Dexterity saving throw. For each target, roll a d8 to determine which color ray affects it.

1. Red. The target takes 10d6 fire damage on a failed save, or half as much damage on a successful one.

2. Orange. The target takes 10d6 acid damage on a failed save, or half as much damage on a successful one.

3. Yellow. The target takes 10d6 lightning damage on a failed save, or half as much damage on a successful one.

4. Green. The target takes 10d6 poison damage on a failed save, or half as much damage on a successful one.

5. Blue. The target takes 10d6 cold damage on a failed save, or half as much damage on a successful one.

6. Indigo. On a failed save, the target is restrained. It must then make a Constitution saving throw at the end of each of its turns. If it successfully saves three times, the spell ends. If it fails its save three times, it permanently turns to stone and is subjected to the petrified condition. The successes and failures don't need to be consecutive; keep track of both until the target collects three of a kind.

7. Violet. On a failed save, the creature has disadvantage on all attack rolls, Strength checks, and Strength saving throws for 1 minute. A creature must then make a Constitution saving throw at the end of each of its turns, ending the effect on itself on a success.

8. Special. The target is struck by two rays. Roll twice more, rerolling any 8.

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 Hydra  regains spent legendary actions at the start of its turn.

Detect. The Hydra makes a Wisdom (Perception) check.

Bite Attack. The Hydra makes a Bite attack.

Flurry Of Crystal and Teeth (Costs 2 Actions). The Hydra attacks out in every direction slamming into its foes. Each creature within 15 feet of the Hydra must succeed on a DC 19 Strength saving throw or take 13 (2d6 + 6) piercing damage, be pushed 10 feet away and knocked prone.