Amphibious. The Greater Hydra can breath both air and water.

Multiple Heads. The hydra has five 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 45 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 25 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.

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

Innate Spellcasting. The Greater Hydras innate spellcasting ability is Intelligence (spell save DC 18). It can innately cast the following spells, requiring no material components:

At Will: Gust of Wind, Thunderwave, Shape Water

4/Day: Fear, Wall of Water, Tidal Wave

3/Day: Storm Sphere, Watery Sphere, Control Water

2/Day: Maelstrom, Conjure Elemental

Immortal Head. One of the Greater Hydras heads (Usually in the center) cannot be removed until all others are removed.

Legendary Resistance (3/day). If the Greater Hydra fails a saving throw, it can choose to succeed instead.