Ancient 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.

Ancient Energy. Whenever a head is severed, roll a d20. On a 12+ the head explodes dealing 1d10 force damage in a 12ft cube. Players hit must take a DC 15 DEX save or be knocked prone.

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

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

The True Hydra can make 1 legendary action every other turn.

• Multi-head Slam. The hydra slams a target with all of its heads, dealing a massive 6d12+10 damage separately to each member of the party. Everyone hit has to take a DC 15 DEX save or be knocked prone.
• Force Blast. The hydra opens one if its mouths wide, firing a purple blast of force energy dealing 1d20+2 force damage.