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 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

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.

Multiattack. The Greater Hydra can make as many Bite attacks as it has heads. It may substitute one Tail attack for one of the Bite attacks but can only make one Tail attack per turn. Melee reach is 15ft.

Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) piercing damage.

Tail. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 15 (3d6 + 9) bludgeoning damage. DC18 Strength Save; On hit the creature must make a Strength saving throw DC18 or be knocked back 10ft and fall prone. On success the creature is only knocked back 5ft and still stand.

Jet Stream. Recharge(5-6). Dexterity Save: DC18; 20d8. The Greater Hydra exhales a jet of icy water and a beam of crackling lightning in a 60-foot line 10 feet wide from two of its heads. Each creature in those areas must make a DC 18 Dexterity saving throw, taking 10d8 cold or lightning damage on a failed save if struck by one beam or 20d8 if hit by both, or half as much damage on a successful one. Being underwater doesn't grant resistance against this damage.

Grappling Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) piercing damage DC18 Strength Save; On hit the creature must succeed a Strength saving throw DC18 or be grappled by one of the Greater Hydras heads. The Greater Hydra can only grapple as many creatures as it has heads.

Legendary Actions. The Greater Hydra can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time, and only at the end of another creature's turn. The Greater Hydra regains spent legendary actions at the start of its turn.

Tail Attack. The Greater Hydra makes one Tail Attack.

Detect. The Greater Hydra makes one Wisdom (Perception) check.

Feast. Melee Weapon Attack: +13 to hit with Advantage, Hit: 35 (4d10 + 18) piercing damage. One creature the Greater Hydra is grappling is assaulted with a flurry of Bite attacks.

Throw. The Greater Hydra releases one creature it is grappling by flinging the creature up to 60 feet away in a direction of it's choice. If the flung creature comes into contact with a solid surface, such as a wall or floor, the creature takes 2d6 bludgeoning damage for every 10 feet it was flung.