Hold Breath. The wyvera can hold its breath for 1 hour.

Multiple Heads. The wyvera has eight 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 wyvera 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 hydra regains 10 hit points for each head regrown in this way.

Reactive Heads. For each head the wyvera has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Wakeful. While the wyvera sleeps, at least one of its heads is awake.

Imperfect Flyby. When the wyvera hits with all its attacks on a turn, it can fly out of reach without provoking opportunity attacks. Otherwise, its movement provokes opportunity attacks as normal.

Multiattack. The wyvera uses its tail sting, then 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.

Tail Sting. Melee Weapon Attack: +9 to hit, reach 20 ft., one target. Hit: 14 (2d8+5) piercing and the target must succeed on a DC18 constitution saving throw or take 27 (6d8) poison damage, half on a successful save