Ancient Elemental 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 35 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra dies.

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.

Elemental Bonus. For each head, they have different resistances and vulnerabilities. 

White - Resistances: Radiant, Vulnerabilities: Necrotic

Black - Immune: Lightning, Resistances: Bludgeoning, Piercing, Slashing from magical and non magical weapons, Vulnerabilities: Poison, Necrotic

Red - Immune: Fire, Vulnerabilities: Ice

Blue - Immune: Ice, Vulnerabilities: Fire

Green - Immune: Poison, Resistances: Necrotic, Acid, Vulnerabilities: Piercing, Slashing from magical and non magical weapons.

Multiattack. The hydra can choose 3 heads to use spells with per turn. Then the hydra makes bite attacks with any heads it hasn't used. Each spell that requires a spell save DC is considered DC 18

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 30 (4d12 + 8) piercing damage.

White head. 1st level (6 spell slots): Cure Wounds, Healing Word

3rd level (5 spell slots): Mass Healing Word

5th level (3 spell slots): Mass Cure Wounds, Raise Dead

7th level (2 spell slots): Regenerate

Black head. 1st level (6 spell slots): Witch Bolt, Chromatic Orb

3rd level (5 spell slots): Lightning Bolt

5th level (3 spell slots): Circle of Power, Hallow

7th level (2 spell slots):  Prismatic Spray

Red head. 1st level (6 spell slots): Searing Smite

3rd level (4 spell slots): Fireball, Melf's Minute Meteors

5th level (3 spell slots): Flame Strike

7th level (2 spell slots): Delayed Blast Fireball, Fire Storm

Blue head. 1st level (6 spell slots): Chromatic Orb, Ice Knife

3rd level (4 spell slots): Hunger of Hadar

5th level (3 spell slots): Cone of Cold

7th level (2 spell slots): Prismatic Spray

Green head. 1st level (6 spell slots): Ray of Sickness, Chromatic Orb

3rd level (4 spell slots): Hunger of Hadar

5th level (3 spell slots): Cloudkill

7th level (2 spell slots): Prismatic Spray

Green head. Once per round, the green head can use Absorb Elements at 3rd level as a reaction.

Protection. If a creature targets the White head with an attack, the Hydra can use its legendary reaction to raise its AC by 3 due to the Black head protecting it. It can do this twice, and this recharges at the start of its next turn.