If One Piece ever actually ends (it won’t but let’s be silly and imagine that it will), then it’s safe to assume that the Ancient Weapons will play a big role in its conclusion. If you don't remember what those are because your brain is too stuffed with the names of all of Big Mom's kids or every minor combatant in the Dressrosa tournament, they are weapons that are capable of mass destruction and they scare the piss out of the World Government. Heck, the World Government has forbid the research of an ENTIRE CENTURY of the One Piece timeline because people might get too curious about the Ancient Weapons.
They have also been teased for hundreds of episodes now and seem to have a strong connection to Poneglyphs, so clearly they are being held back for some grand purpose. But what are they exactly?
We know about two of them: Pluton and Poseidon, the former being some kind of amazing ship capable of destroying entire islands. CP9 was tasked with retrieving plans for it for Spandam during the Water 7/Enies Lobby arcs, but they were thwarted when Franky pulled a dracarys on everyone and torched the blueprints. Still, the original Pluton ship remains out there somewhere, ready to unleash its awesome power on the world.
But who will control it? Also, why the heck are people getting so bent out of shape over a ship? There must be millions of sea vessels in the world of One Piece. What does Pluton have that every other shipwright hasn’t figured out yet? What, did its creator put the mast on the bottom of the ship or something?
Personally, I think it’s because Pluton is a huge submarine. In One Piece, ships can be coated in a special resin that allows them to “sail” underwater but the process is incredibly dangerous, to the point that the majority of underwater ships tend to sink and crash as soon as they go under the waves. But now imagine a gigantic battle cruiser the size of an island being able to stay underwater for weeks and navigate not by riding the Sea Currents, but with an engine.
We’ve seen this on a smaller scale with the Heart Pirates’ Polar Tang or with Franky’s Shark Submerge. But this is “modern” OP technology. Pluton is supposed to be hundreds of years old, so it could very well be some kind of “lost tech” submarine with incredible firepower and propulsion the likes of which we’ve never seen. If something like that existed in the OP-verse, it would have the power to upset the entire world order.
And then there's Poseiden, the second Ancient Weapon:
Poseidon isn’t actually a thing. It’s a mermaid with the power to control Sea Kings, some of the most powerful animals in One Piece. This brings me to the third Ancient Weapon: Uranus.
It hasn’t yet made an official appearance on the show or in the manga, and seeing as one Weapon was an invention and the other a living person, it could literally be anything, right? Well, no. It still needs to have a connection to a great source of power. Pluton is probably the most powerful manmade thing in the world and Poseidon controls the most powerful things in nature. What other source of “power” is there in all of One Piece? Devil Fruits. A submarine/magic ship/whatever sounds great but how would it fare against Kizaru’s laser beams? Similarly, Sea Kings can probably be repelled/roasted crispy by a man who can turn himself into magma. So a third Ancient Weapon would need to be something that could stop One Piece's army of evil middle-aged men with goatees.
That's why the third Ancient Weapon could be, like, a gigantic Sea Stone that creates a massive force field that nullifies all Devil Fruit abilities around it. But that still won’t do you much good against people like, say, Byrnndi World, who can just throw a huge boulder at you. So what if the third weapon is also a Devil Fruit? A Devil Fruit, more specifically, that we’ve already seen?
Think about it. A Devil Fruit Ancient Weapon would need to be able to nullify the abilities of other fruit users, dispel direct physical attacks, and be able to grow in strength. Doesn’t that just remind you of Blackbeard’s Dark-Dark Fruit? It allows its user to create a dimension of darkness where they control all the gravity, sucking in and stopping any attack that may come their way. But it doesn’t just gobble up physical objects and people. It also sucks out a person’s Devil Fruit ability, as we’ve seen when Blackbeard depowered Ace, leaving him drier than an attack by Crocodile.
More importantly, the Dark-Dark Fruit can permanently take your power and give it to Blackbeard. After Whitebeard’s death, we saw Marshall taking Eddie’s Earthquake Quirk… Sorry, wrong anime. He took Newgate's Tremor-Tremor Fruit power, becoming the first Dual Fruit user in all of OP. And nowhere does it say that Blackbeard can’t do it with more fruits. Yeah, he now has to use one hand for each power but maybe it also works on other appendages. If he stole Kizaru’s power, maybe he could shoot laser beams from his leg. If he got his hands on the Flame-Flame Fruit, he could probably crap fire. Look, we were ALL thinking it, right? Right? We were all thinking it. Please, someone else be thinking it.
The point is, the Dark-Dark Fruit could very well be an Ancient Weapon. It would make sense from a storytelling point of view. Currently, Blackbeard has one Ancient Weapon and Shirahoshi (the current Poseidon) could end up in a tentative alliance with the World Government, potentially leaving Pluton free to be claimed by the Straw Hats and ultimately leading to a final battle where Ancient Weapon will be pitted against Ancient Weapon. And that would be pure awesomeness.
Do you think that’s how it will all play out? Do you think Uranus is something totally different? Let me know in the comment section.
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