Wrath of the Sea Hydra (Mythic Trait; Recharges after a Short or Long Rest). If Eudoxos is reduced to 0 hit points, she doesn't die or fall unconscious. Instead, she regains 260 hit points. In addition, she gains two heads, which are Gargantuan creatures that appear in unoccupied spaces within 60 feet of her, sharing the same hit points. Whenever she takes an action, it can originate from any of her heads. While she has more than one head, Eudoxos has advantage on saving throws against being blinded, deafened, stunned, or knocked unconscious. Whenever Eudoxos takes 50 or more damage in a single turn, one of her heads dies. If all her heads die, Eudoxos dies. At the end of her turn, she grows two heads for each of her heads that died since her last turn. Eudoxos regains 25 hit points for each head regrown in this way. 

Magic Resistance. Eudoxos has advantage on saving throws against spells and other magical effect.

Legendary Resistance (3/Day). If Eudoxos fails a saving throw, she can choose to succeed instead.

Innate Spellcasting. Eudoxos' spellcasting ability is Charisma (spell save DC 24?). She can innately cast the following spells, requiring no material components:

At will: control water, wall of water

Heartfettered: Eudoxos has a section of scarred and shattered scales across her chest. An attack that does at least 20 damage directed at it will reveal the Heartfetter, the artifact used to bind her. Once revealed, the Heartfetter can be removed with a DC25 athletics check. This check decreases by 1 with each attempt.

Multiattack. Eudoxos can use her Frightful Presence. She then makes as many bite attacks as she has heads and one tail attack, which she can replace with one use of Fling. She can replace any of her bite attacks with a water jet attack..

Bite. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 18 (2d10 + 7) piercing damage plus 9 (2d8) cold damage.

Water Jet. Ranged Weapon Attack: +14 to hit, range 120 ft., one target. Hit: 21 (6d6) bludgeoning damage, and the target is knocked prone if it fails a DC 22 Strength saving throw.

Tail. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 16 (2d8 + 7) bludgeoning damage. In addition, the target must succeed on a DC 22 Strength saving throw or be grappled (escape DC 22). Until this grapple ends, the target is restrained. Eudoxos can only grapple one target at the same time this way.

Fling. One Large or smaller object held or creature grappled by Eudoxos is thrown up to 60 feet in a direction of her choice and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 22 Dexterity saving throw or take the same damage and be knocked prone.

Frightful Presence. Each creature of Eudoxos's choice that is within 120 feet of her and aware of her must succeed on a DC 22 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Eudoxos's Frightful Presence for the next 24 hours.

Water Breath (Recharge 5-6). Eudoxos exhales pressured water in a 90-foot line that is 10 feet wide. Each creature in that area must make a DC 22 Dexterity saving throw, taking 77 (22d6) bludgeoning damage on a failed save and being pushed up to 30 feet away of Eudoxos on a failed save, or half as much damage and not being pushed back on a successful one.

Tsunami (Recharges after a Long Rest). Eudoxos casts the Tsunami spell, without requiring concentration (save DC 22). She is immune to its effects.

Eudoxos 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. Eudoxos regains spent legendary actions at the start of her turn.

Detect. Eudoxos makes a Wisdom (Perception) check. She makes this check with advantage if she has more than one head.

Move. Eudoxos moves any number of heads up to her movement speed.

Tail Attack. Eudoxos makes a tail attack or uses her Fling.

Crush. One creature that Eudoxos is grappling is crushed for 23 (3d10 + 7) bludgeoning damage.

If Eudoxos's mythic trait is active, she can use the options below as legendary actions for 1 hour after using Wrath of the Sea Hydra.

Water Prison (Costs 3 actions). Eudoxos casts the Watery Sphere spell on three targets (save DC 18).