Acidic Blood. When the abomination takes piercing or slashing damage, each creature within 5 feet of the hydra takes 9 ( 2d8 ) acid damage.   Amphibious. The abomination can breathe air and water.   Cold Susceptibility. The abomination has disadvantage on saving throws against attacks that do cold damage. On a natural 1 the abomination is stunned until the end of its next turn.   Cloying Miasma. The abomination is surrounded by a noxious stench. At the start of the abomination’s turn, any creature within 5 feet of it must succeed on a DC 19 Constitution saving throw or be poisoned until the start of the abomination’s next turn.   Legendary Resistance (1/Day). If the abomination fails a saving throw, he can choose to succeed instead.   Multiple Heads. The abomination has nine heads. While it has more than one head, the abomination has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious.   Whenever the abomination takes 35 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 abomination regains 10 hit points for each head regrown in this way.   Reactive Heads. For each head the abomination has beyond one, it gets an extra reaction that can be used only for opportunity attacks.   Wakeful. While the abomination sleeps, at least one of its heads is awake.