The Ancient Weapons form one of the most important plot points in One Piece. As the name suggests, these weapons were created in ancient times, and they have enormous destructive power. In fact, it has been said that the ancient weapons have the power to destroy the entire world if they fall into the wrong hands.
Such weapons of mass destruction were essentially created by none other than the people of the Ancient Kingdom, who possessed incredible scientific prowess. While the ancient weapons were used long ago in the One Piece world and were responsible for sinking it about 200 meters below the sea, they are still active right now, waiting to be used, and only a few individuals know their whereabouts.
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The Royal Family of Fishman Island
The royal family of Fishman Island consists of King Neptune, his three sons and one daughter. Neptune's very own daughter, Shirahoshi, is an ancient weapon, who also goes by the name of Poseidon, the mermaid princess. Of course, the family knows about Shirahoshi's true nature and that she will one day help reshape Fish-Man Island when Joy Boy is resurrected.
All of them have been waiting for her powers to awaken, and it successfully happened in the Fishman Island arc, especially towards the end, when Shirahoshi was able to order the Sea Kings to stop Noah from destroying the entire island.
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Caribou
Caribou's main role in the story was to get key information to a specific individual that will be touched on later. Caribou was in the Fishman Island arc, and while he didn't really play much of a role, he successfully eavesdropped on the conversation between the Straw Hat Pirates and King Neptune. It is through this that he found out that Shirahoshi was an ancient weapon and that she has enough power to destroy the entire world.
Caribou was then seen in Wano once more. There he heard the conversation about the old weapons again. This time he found out that the location of Pluton is under Wano Country and all he needs to do is open Wano's borders to unleash the weapon.
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Blackbeard (via Caribou)
Blackbeard was not aware of the location of any ancient weapon until recently. But Caribou recently met the Blackbeard Pirates and had important information to give them. In fact, it was revealed to fans that Caribou was actually a huge fan of Blackbeard and that he was the main reason he even set sail in the first place.
Now Caribou will sell all the secrets to Blackbeard and reveal to him the location of both Pluto and Poseidon. This information will have major repercussions that we absolutely cannot wait to see in the latter half of the Final Saga of One Piece. It is also very likely that his pirate crew, along with Blackbeard, are also privy to this key information about the ancient weapons. What they will do with this information going forward remains to be seen. It is quite likely that they will invade Wano Country or at least attack Fishman Island. Either case puts them in direct contention with the Straw Hat Pirates, which is something we absolutely cannot wait for as fans.
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Straw Hat Pirates
The Straw Hat Pirates don't necessarily care about the ancient weapons at all. However, that doesn't mean they don't know any weapon. In the Wano Country arc, Nico Robin clearly informed the Straw Hat Pirates that Pluto is actually sleeping under Wano. Robin asked Luffy if he wanted the Weapon, to which he replied that he didn't care.
Nevertheless, Luffy and all the other members of the Straw Hat Pirates now clearly know the location of the Ancient Weapon Pluton. This is very exciting and will definitely come into play later. At some point, Luffy is bound to use the ancient weapons, and this has also been foreshadowed through the Fishman Island arc, which officially states that he will destroy the island once and for all. Considering that only weapons like the ancient weapons have the ability to accomplish something like this, it's highly likely that Luffy will use them sooner or later.
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Nico Robin (Poseidon)
Nico Robin is part of the Straw Hat Pirates and should therefore already be included in the previous post. She has information on more than one ancient weapon that the other members of the crew do not. Nico Robin also knows Poseidon's location on top of Pluto. She only disclosed the location of Pluto to the crew, while the knowledge of Poseidon remains with her.
Nico Robin found out this information in the Fishman Island arc, and there it was pretty clear for fans to see that she read the Poneglyph in the Sea Forest and found out the truth about the weapon. She also knows about Joy Boy and his return, and it will be interesting to see what this knowledge will do for Robin later.
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Imu and The Elders
Clearly, Imu and the five elders are aware of the ancient weapons and their locations. Fans know that the Elders wanted to see Wano's borders opened specifically because of the location of the Ancient Weapon Platoon there. More importantly, they also have one of the ancient weapons at their own disposal. Ancient Weapon Uranus was recently revealed to be in the possession of Imu, and the Elders used their powers to destroy Lulusia, after the Mother Flame was stolen from Vegapunk.
Now that Vegapunk Lilith has officially sided with the World Government and is officially producing more Mother Flame, it's only a matter of time before the ancient weapon Uranus is loaded and ready to go again. With more Mother Flame, this weapon will become a nightmare for all those who oppose the World Government, as it can sink islands and, with it, the world of One Piece even further.
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Kozuki Sukiyaki (& Momo and Hiyori, potentially)
Kozuki Sukiyaki is yet another powerful individual who fans know holds the secrets of the ancient weapons. As the Daimyo of Wano, he knows where Pluton is located, and he was the one who told Robin that this weapon is under Wano's land. He may have also passed this information on to Momo and Hiyori, especially now that the former had become Shogun.
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Trafalgar law
When Kozuki Sukiyaki revealed the location of the Ancient Weapon Platoon, Trafalgar Law was also present at the scene. As such, he is also aware of the location of this ancient weapon. That said, his interest is more in the history of D, rather than why these weapons exist and what their purpose is. Still, this information may come in handy for him later in One Piece.
All the latest chapters of Eiichiro Oda's One Piece manga are available to read via Manga Plus and Viz Media.
