Adult Tiamat 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. 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.

Chromatic dragon heads. The Tiamat hydra has 5 out of its six heads as chromatic dragon heads and resembles Tiamat, through the sixth head is an ordinary hydra head and has no breath weapon, but it has an intelligence of 10(+0) and controls the body usually, the other heads have an intelligence of 2(-4) and have a little sway in where the body goes, if they can mentally overpower the sixth head. whenever a dragon head is cut off two of same chromatic/hydra dragon head regrows from its place as it would normally.

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

Bite. Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 30 (5d10 + 5) piercing damage.

The Tiamat Hydra has legendary actions equal to the number of heads it has, if it were to grow more heads it would gain extra legendary actions.

Special heads(1 action).  The Tiamat Hydra can use its breaths as an legendary actions for each of its heads. 1st head has poison breath(2d6 poison damage, 15 ft cone/con save), 2nd head has fire breath(2d6 fire damage, 15 ft cone/dex save), 3rd head has acid breath(2d6 acid damage, 5 by 30 ft line/dex save), 4th head just uses a bite action, 5th head has Ice breath(2d6 cold damage, 15 ft cone/con save). 

Bite. Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 30 (5d10 + 5) piercing damage.