Inspired by: https://www.dndbeyond.com/monsters/1052965-stormhydra
Reflecting Scales: On a successful saving throw against any spell that targets Stormhydra directly is reflected back at a random target.
Multiple Heads. The hydra has 10 heads. While it has more than one head, the hydra has advantage on saving throws against being charmed, blinded, defended, frightened, stunned, and knocked unconscious.
Whenever the hydra takes 30 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 or necrotic damage since its last turn. The hydra regains 20 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.
Electrified Form. A creature that touches the hydra or hits it with a melee attack while in water and within 5 feet of it takes 5 1d10 lighting damage. In addition, the hydra can enter a hostile creature's space and stop there. The first time it enters a creature's space on a turn, that creature takes 5 1d10 lighting damage and must make succeed a DC 13 Constitution saving throw or become stunned for one round.
Can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Spent legendary Actions are regained at the start of each turn.
Reflected Past. If the Stormhydra fails a saving throw, it can choose to succeed instead. (Legendary Resistances x3)
Swallow. When the Stormhydra makes one bite attack against a Large or smaller target; If the attack hits, the target is also swallowed. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the stormhydra, and it takes 21 6d6 lighting damage at the start of each of the hydra's turns.
If the hydra takes 30 damage or more on a single turn from a creature inside it, the hydra must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the hydra. If the hydra dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.
Thunder Step. 90 feet Instantaneous. You teleport yourself to an unoccupied space you can see within range. Immediately after you disappear, a thunderous boom sounds, and each creature within 10 feet of the space you left must make a Constitution saving throw, taking 3d10 thunder damage on a failed save, or half as much
Ocean, Sea, Lakes, Elemental Plane of Water, Elemental Plane of Air