Cobalt 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 30 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 20 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.

Adaptive Resilience. The Hydra alternates between magic and physical resistance based on what damage type it took at the end of a creatures turn.

Decimating Breath. The Hydra fires a concentrated beam in a 10 by 25 line dealing (2d6 fire damage) * Each head still alive. Each target must make a DC 16 Dex saving throw, on a success they take half as much. This can only be recovered from Magic Ingestion.

Magic Ingestion.  The first time the hydra makes a bite attack the target must succeed on a DC 16 dex saving throw or be [Tooltip Not Found] till the end of the hydras next turn. If the head eating the target is killed, the player falls out of it prone(when players do damage it's always the head devouring the target). Otherwise the player takes (2d6) magic damage and the Hydra regains Decimating Breath and the target is spat 10ft away from the Hydra, Prone.

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

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

Decimating Breath. 

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