Beefy 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 50 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.

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

Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 11 (2d8 + 7) piercing damage plus 1d10 poison damage. The target must succeed on a DC 15 Constitution saving throw or be poisoned for 1 minute. The creature can repeat the saving throw at the end of each of its turns, ending the effect on a success. While the target is poisoned, it takes 1d6 poison damage at the start of each of its turns.

Gore. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 11 (2d8 + 7) piercing damage plus 8 (2d8) bludgeoning damage. 

This 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.

Attack. The hydra makes one bite or gore attack.

Move. The hydra moves up to half its speed.

Stomp (Costs 2 Actions). They hydra stomps its massive feet causing quakes. Each creature within 20 feet of the hydra must succeed on a DC 20 Dexterity saving throw or take 13 (2d6 + 7) bludgeoning damage and be knocked prone.