Corrupted Hydra

Multiple Heads. The hydra has twelve 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 20 or more damage in a single turn, one of its heads dies. It can lose multiple heads in one turn. 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 more than 10 radiant damage since its last turn. The hydra gains 10 temporary 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.

Shadow Stealth. While in dim light or darkness, the hydra can take the Hide action as a bonus action.

Sunlight Sensitivity. While in sunlight, the hydra has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

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

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage and 5 (1d8) necrotic damage. A hit target can't regain hit points until the end of the hydra's next turn. If this attack hits an undead target, it also has disadvantage on attack rolls against the hydra until the end of the hydra's next turn.