Amnesia 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, deafened, 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.

Regeneration. The Amnesia Hydra regains 20 hit points at the start of its turn if it has at least one head remaining. If the hydra takes radiant damage or damage from a critical hit, this trait doesn't function at the start of the hydra's next turn.

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 attacks as it has heads. You can replace 1 of the Amnesia Hydra's attacks with a Memory Drain, Echoing Roar or Siren's Song.

Bite. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 10 (2d10 + 7) piercing damage.

Memory Drain. The Amnesia Hydra targets one creature within 60 feet. The target must succeed on a DC 20 Wisdom saving throw or lose all memory of the past hour. On a failed save, the target also takes 6d10 psychic damage.