Aspect of poseidon

Legendary Resistance (3/day). If Poseidon fails a saving throw, he can choose to succeed instead.

Godly Weapon. Poseidon’s weapon attacks are magical.

Immortality: If Poseidon dies, he comes back to life in 1d4 days.

God of the seas. ( Mythic Trait; Charges after a Short or Long Rest). If Poseidon would be knocked unconscious , he now grows to gargantuan and is made of water , his trident knows grow with him  and his current hit point total resets to half his hit point maximum; his trident know deals a extra 1d10 damage and he can now use the options in the "Mythic Actions" section for 1 hour. Additionally The spell storm of vengeance is casted with a range of 600ft centered on Poseidon; however Poseidon is immune to the effect of the spell.

 Innate Spellcasting. Poseidon’s innate spellcasting ability is wisdom (spell save DC 24, +16 to hit with spell attacks). He can innately cast the following spells, requiring no material components:

• At will:control water, dominate monster (must have a swimming speed)
• 3/day each: tidal wave, wall of water, watery sphere
• 1/day each: earthquake, maelstrom, tsunami

Summon sea creatures (1 /Day). Poseidon slams his trident on the ground, any creature with in a hundred feet of the ground must make a (DC 18) Strength save or be knocked prone and take 4d10 bludgeoning damage. As geysers erupt from the ground, ( If the are allows it The room begins to fill with water in a area of 300ft wide and 10 ft high. 

Poseidon summons 2d8 Reef sharks, 4d6 Sahuagin Baron, 1d4 sea hags, 1d4 poisons snakes, , or one storm lord.

A summoned beats appears in an unoccupied space within 60 feet of its summoner, acts as an ally of its summoner, and can't summon other demons. It remains for 1 minute, until it or its summoner dies, or until its summoner dismisses it as an action.

Multi-attack: Poseidon can make 2 attacks with his trident.

Trident of the seas. Melee Weapon Attack: + 15 to hit, reach 10 ft.,1 target. Hit: 24 (3d10 + 9) bludgeoning + (2d4) radiant damage.

Trident of the seas. ranged Weapon Attack: + 9 to hit, reach 120/300 ft.,1 target. Hit: 24 (3d10 + 9) piercing + (2d4) radiant damage.

Bolt of the sea god (Recharge 5-6): Poseidon shoot a bolt of energy any were in a 400ft range from his trident, any creature in a 30ft range of the bolt must make a DC 20 Dexterity saving throw, taking 72 (16d8) radiant damage on a failed save, or half as much damage on a successful one.

Recall. If Poseidon throws his trident he can summon it back to his hand as a bonus action.

Watery Form. Poseidon momentarily assumes the form of water, reducing the damage of incoming attacks (including area of effect attacks) by half. This does not apply to lightning damage

 Bolt  (Costs 2 Actions). Poseidon uses his bolt of the gods. 

If Poseidon's mythic trait is active, it can use the options below as legendary actions for 1 hour after using God of the sea. He has 3 actions

Conjure water elemental (Costs 1 Actions). Poseidon summons one water elemental with in a hundred feet of him.

Earth-shaker: ( Cost 2 action) Poseidon cast Earthquake.

Ruler of the seas: ( Cost 3 actions) Poseidon cast Tsunami.