One Piece is filled with mysteries, and it is extraordinary that even after a thousand chapters, Eiichiro Oda has managed to keep several mysteries still under wraps. One of the biggest mysteries in One Piece are the Ancient Weapons. There are three Ancient Weapons - Pluto, Poseidon, and Uranus. All three are considered to be capable of causing widespread destruction. It was during the Alabasta arc that the Ancient Weapons were properly mentioned for the first time.
Crocodile wanted to get his hands on Pluton, which is considered to be the greatest warship in history. It is deemed to be powerful enough to decimate an entire island. With such ridiculous power, there is no wonder why multiple characters are trying to get their hands on one of the Ancient Weapons. So far only one Ancient Weapon has been shown, and it is Poseidon. Unlike Pluton, Poseidon is actually a living being. The current wielder of this power is Shirahoshi, who is the princess of Fish-Man Island.
The Prophecy
Before Shirahoshi was even born, it was prophesied by Shirley that Neptune's daughter would be born with the power to potentially destroy the world. The poneglyph on Skypiea talked about Poseidon, which led the Roger Pirates to Fish-Man Island. After arriving at Fish-Man Island, Oden read the two poneglyphs and one of the pongelyphs contained Joyboy's apology to the people of Fish-Man Island as he had failed to fulfill his end of the promise.
As Shirahoshi was not born at this time, Roger could not complete Joyboy's dream either. By the time Shirahoshi was finally born, Roger had been executed by the World Government. It took Shirahoshi several years to awaken her dormant power. When she saw her mother being threatened by Mjosgard, Shirahoshi began to cry, and her voice attracted the massive Sea Kings. When Otohime found out about Shirahoshi's power, the former declared that it is the strongest power in the world. Otohime realized that if this power fell into the wrong hands, it could lead to a disaster, so she did everything to protect her daughter.
Years later, when the Straw Hats visited Fish-Man Island, Shirahoshi had grown up, but she was still unable to control the power, which was lying dormant inside her. She did have a moment, where she summoned the Sea Kings to take away Noah and prevent it from crashing into Fish-Man Island.
The Power To Change The World
Poseidon has the power to talk to the Sea Kings and give them commands. Sea Kings are gigantic, and they can easily destroy a huge ship. Poseidon's power is intended to be used to carry Noah to the surface in order to keep her promise to Joy Boy. However, the promise could not be fulfilled and for years the generations, the power of Poseidon was passed down to the mermaids born into the Royal Family. However, no one of able to awaken the power until Shirahoshi. Shirahoshi is not a violent person, so it is logical to assume that she would not use her power to destroy islands.
However, things would be different if Shirahoshi is captured and used by someone. The Sea Kings are not stupid in fact, they do not destroy anything without first deliberating about the possible harms associated with the order, but if Shirahoshi's life is at risk, they will have no choice. With a massive army of Sea Kings, Shirahoshi can easily take down entire islands. The tremendous number of the Sea Kings would be enough to crush almost everyone. In addition to being powerful, their ability to think makes them even more deadly. If she remains on Luffy's side, they can finally accomplish what Joy Boy wanted to do all those years ago. However, if she is kidnapped by the World Government or any other villain, then it could spell the doom of the world.
Is She The Strongest Character?
As an Ancient Weapon, Shirahoshi is extremely dangerous to the safety of the World Government. During the Reverie arc, Im was seen with the poster of Shirahoshi, which means that they have figured out that Shirahoshi is Poseidon.
While Shirahoshi has ridiculous power, she does not have the strength to fend for herself, and this was highlighted countless times on Fish-Man Island. She needs someone to protect her, so if a person is sent to assassinate her and if she does not have anyone around her at that time, Shirahoshi will inevitably die. So, it is quite difficult to call her the strongest character, instead, it would make sense to call her the most lethal weapon in One Piece.