Poseidon

Poseidon (ポセイドン) is one of the three ancient weapons, alongside with Pluton and Uranus. 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 colloquially as the "Mermaid Princess" (人魚姫). Currently, Shirahoshi has inherited Poseidon's power and her name.

Abilities and Powers
Poseidon has the power to communicate with and control Sea Kings, an ability that even the most powerful merfolk cannot do. This is considered to be a powerful weapon because of the Sea Kings' destructive potential to sink islands with ease, and to control such a force is considered to be one of the most fearsome abilities in the entire world. While guided by the power of love, it can save thousands of lives, but with evil intentions, it can sink everything into the ocean. The Sea Kings refer to the Mermaid Princess as their "queen".

The current Mermaid Princess, Shirahoshi, has demonstrated, when her powers first awakened at age six, no control over her own abilities and will inadvertently summon Sea Kings when she cries out loud, which is very risky to her kingdom. Ten years later, during the battle against the New Fishman Pirates, she has fully awakened the ability and seemingly 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 power was fully awakened during the fight with the New Fishman Pirates, which Neptune notes is not a good thing.

Legend


While normal merfolk are able to communicate with fish via sonar waves, and a certain king can even speak with whales, they cannot reach the monstrous Sea Kings; only a certain mermaid within the Ryugu Kingdom's royal bloodline with the rare ability may speak to Sea Kings and command them.

It is said that every few centuries, a mermaid is born with the unbelievable ability to control Sea Kings. When the time comes, someone will come to guide the said legendary Mermaid Princess to change the world forever.

Reactions to the Legend
During the Void Century, a Mermaid Princess had cooperated with Joy Boy in an attempt to bring the giant ark Noah to the surface world through the princess' power to command Sea Kings. Joy Boy failed to keep his end of the promise, but he left it to the future generation to finish it, and apologized on a poneglyph. The World Government, in fear of the ancient weapons, banned the research of poneglyphs, including anything about Poseidon, which is the alternate name of the Mermaid Princess.

Vander Decken entered the sea floor in his pursuit of the legendary Mermaid Princess' powers. He died on Fishman Island before he could realize his ambitions, but his descendants still continue his legacy of searching for the princess.



Eight years before the start of the series, Princess Shirahoshi had unwittingly demonstrated the ability to summon Sea Kings when her mother was in danger of being shot by World Noble Mjosgard. Her cries summoned a group of the sea monsters, allowing her mother and Vander Decken IX to find out of her ability. At this point, Decken deduced that Shirahoshi is the Mermaid Princess of legend that his family has been searching for and decided to marry her to gain access to her fearsome abilities (as well as her unparalleled beauty), leading him to send her threatening marriage proposals for the next ten years. Otohime, knowing that such powers can be as beneficial as deadly, had her three sons train hard to become warriors in order to protect Shirahoshi when the time comes.

It seems that Shirahoshi eventually discovered her ability to speak to Sea Kings, as she did not show shock when Jinbe revealed it during his explanation of Fisher Tiger's and Otohime's past. Decken, desiring her powers, attacked her by throwing at her one weapon after another, considering it his way of showing affection and proposal for marriage. Upon realizing that she did not love him at the very least, he opted to kill her in order to prevent her from falling into the arms of another man.

Hody Jones is also aware of Shirahoshi's ability and deems her a threat to his plan of taking over Ryugu Kingdom. He teamed up with Vander Decken IX in hopes that she would be eliminated.

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 the Roman name of Poseidon and Otohime is the daughter of the Japanese sea-god Ryūjin.

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