Poseidon

Poseidon (ポセイドン) is another of the ancient weapons with intentions like that of Pluton. It is a mermaid who has the power to communicate with Sea Kings.

Appearance
Rather than being an inanimate object like usual weapons, Poseidon was a mermaid belonging to the Ryugu Kingdom's royal bloodline. After she died, her name was treated as a title given to a mermaid (belonging to the Ryugu Kingdom's royal bloodline) with the same power she possessed when alive, known as the "Mermaid Princess". Currently, Shirahoshi has inherited both Poseidon's power & her name.

Abilities and Powers
Poseidon has the power to communicate with and control Sea Kings. That power is considered a weapon because Sea Kings can tear islands apart and sink them into the ocean, meaning whoever can control their power is a threat to the world. However, with the guidance of love, the power can be used to save thousands. Shirahoshi currently possesses this power, and during the battle between the Straw Hat Pirates and the New Fishman Pirates, she fully awakened the ability and gained control of it.

History
Poseidon lived during the Void Century and made a bargain with Joy Boy. She would use her powers to command the Sea Kings to bring Noah to the surface of the world and activate its intended mission of which the details would be revealed when raised. However, Joy Boy failed in his commitment to Poseidon. On the Ryugu Poneglyph located in Sea Forest is a letter of apology from Joy Boy to Poseidon, apologizing for breaking his end of the bargain. The Shandora Poneglyph also contains information about Poseidon.

Even after her death, her power did not die and Poseidon's ability was passed through the generations of the royal family, although never awakened. Meanwhile, Fishman Island attempted to keep its end of the bargain despite Joy Boy breaking his. The World Government spent the next few centuries trying to avoid the awakening of the Ancient Weapons by outlawing the reading of the poneglyphs. Despite their efforts, Poseidon's power resurfaced in Shirahoshi, the daughter of Neptune. Her powers fully awakened during the fight with the New Fishman Pirates, which Neptune notes is not a good thing.

Trivia

 * In Greek mythology, Poseidon (Ποσειδών) is the god of the sea, water and earthquakes. This notably keeps in theme with the rest of the Ryugu Kingdom royal family; Neptune is named after the God from Roman mythology of the same name and Otohime, who is named after the daughter of the Japanese sea god Ryūjin.

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