Multiple Heads

The creature has X heads. While it has more than one head, the creature has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious. Whenever the creature takes Y 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 Z 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 A hit points for each head regrown in this way. (Hydra, pp. 190, MM)