Adult hydra dragon

amphibious. the hydra can breath air and water.

Acidic blood. when the hydra takes piercing or slashing damage, each creature within 5 feet of the hydra takes 3d6 acid damage.

legendary resistance. if the hydra fails a saving throw, it can choose to succeed instead.

multiple heads.  while the hydra has more then one head, the hydra has an advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious. whenever the hydra takes 35 or more damage in a single turn, one of it's heads dies. if all of its heads die, the hydra dies. at the end of its turn, it grows two more heads for each of its heads that died since its last turn, unless it has taken fire damage since its last turn. the hydra dragon 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.

siege monster.  the hydra dragon deals double damage to objects and structures.

wakeful. while the hydra sleeps, at least one of it's heads is awake.

multiattack. the hydra makes as many bite attacks as it has heads and two claw attacks and one tail attack.

Bite. melee weapon attack : +8 to hit, reach 10 ft., one target. hit : 2d10 piercing damage + 2d6 poison damage.

Claw. melee weapon attack : +8 to hit, reach 5 ft., one target. hit : 2d8 slashing damage.

Tail. melee weapon attack : +8 to hit, reach 20 ft., one target. hit : 3d6 bludgeoning damage.

Constrict. melee weapon attack : +8 to hit, reach 20 ft., one target. hit : 3d8 bludgeoning damage, and the target is grappled until escape, escape str saving throw of 14 or higher taking another 3d8 bludgeoning on a failed save, grapple ends when target escapes.

Acid breath ( middle head only ). the hydra exhales acid in a 60 foot line that is 5 feet wide. each creature in that line must make a dex saving throw of 16 or higher taking 12d8 acid damage.

Poison breath ( any head ). the hydra exhales poisonous gas in a 60 foot cone. each creature in that area must make a con saving throw of 16 or higher taking 12d6 poison damage.

the hydra dragon can make 3 legendary actions.

wing attack. the hydra beats its wings. each creature within 15 feet of the hydra must succeed on a dex saving throw of 15 or higher taking 3d8 bludgeoning damage and be knocked prone.

flurry of bites. melee weapon attack : +9 to hit, reach 15 ft., one target. hit : 2d6 piercing damage + 4d8 poison damage. the hydra bites as many targets as it can