This is an attempt to explain some already known things about the 2 weapons and deduce the use of the final one. It’s a long one but bear with me. But first, some context.

Ancient Weapons and the path to reach them:

• Pluton (Rules the underworld, also buried deep under old Wano) : A “ship” (or anything that the shipbuilders from Tom’s workers can build) which is rumored to obliterate an island with one shot. A Poneglyph in Arabasta tells you its location in Wano.

Note: Arabasta has been protecting the Poneglyph, Cobra wonders why it’s hidden if it can tell the true history and the Nefertari family refused to join the first 20 Kings, making them traitors to the Word government and an ally of the Ancient Kingdom, he asks them about Poneglyphs and the rest is history. Wano is another ally trusted with a red Poneglyph and Pluton.

• Poseidon (Rules the Seas, a Mermaid in Fishman Island): One of the Merfolk that is born with the power to command Sea Kings.

Note: Fishman Island is another ally entrusted with another red Poneglyph and a special Poneglyph which is an apology to Poseidon from back then by Joyboy.

• Uranus (Personification of the sky): Nothing is known so far about it, some people say it’s the one Im uses, but for the sake of this post let’s assume it’s not that.

Pluton, Poseidon and the Fishmen dream:

In Fishman Island we see how many people dream of living in the surface : Shirahoshi wants to see the sun and forest, Arlong and co. recreated Sabaody Park in the East Blue and Otohime’s whole fight to get signatures for the petition.

Neptune says Otohime’s dream is the same their ancestors pursued and spans hundreds of years, and at the end of the fight Fukaboshi says this is the start of his mother’s dream.

Notice how Noah is present in both backgrounds, Noah is the “ship of promise” and Joyboy apologizes to Poseidon for not fulfilling a promise.

According to Shyarly Luffy will destroy Fishman Island and according to the Sea Kings, Noah was made to be pulled by them.

Many of you are aware this already but it’s worth a mention: This all points to the likely possibility of Luffy using Pluton to destroy the Red Line while the Fishman Island residents ride the Noah and get transported to the surface by Sea Kings under Poseidon’s command

Giants and Sky People:

1. Giants:

From the latest chapters, we see how giants carry Ohara’s books to Elbaf with Saul. Add to that the facts that Saul is a D., that he says he’s not from Elbaf, the giant Robot from 900 years ago resembling Oars and the giant straw hat from back at the Reverie it became more likely that giants could be potential allies of the Ancient Kingdom as well.

In Big Mom’s flashback, Jarul says they must not forget they are warriors, this seemed like a reference to how they’re Vikings at the time but it could also mean that they know something about the past and are making sure to stay as warriors in preparation for the final war.

2. Sky People:

In Enel’s trip to the Moon, he finds out the Sky People originally came from the Moon City “Birka” which shares the same name as Enel’s homeland, the same people flew to the “Blue Planet” for resources (notice the tears, this is most likely a one way trip and a farewell considering the deserted state Enel finds the city in).

The most important thing here is how we see Birkans with their wings pointing down, Shandorians (Shandora people and Shandian ancestors) wearing animal skin like the Shandians and what is assumed to be Skypieans by process of elimination.

This all means that long ago, all those 3 peoples where living together in harmony.

3. Giant Jack:

Remember the beanstalk in Upper Yard? When most of us saw it for the first time we all thought of Jack and Beanstalk, the fairy tale involving giants. We know for a fact that Giant Jack was there long before Upper Yard as it was the reason the Golden Belfry got lost and we never see its roots, so what does this have to with anything?

Robin finds out that Shandora thrived 1100 years ago and was destroyed 800 years ago while protecting the Poneglyph, we know Shandora was home to winged people which are ancestors to Shandians (Chandra is also the Hindu god of the Moon, who is associated with the Night, Plants and Vegetation).

When the Sky People arrived from the Moon, they possibly settled in Jaya (all 3 “sub-races”) then a few centuries later, the Ancient Kingdom rose, made allies with multiple islands through their ideals including Fishmen/Merfolk, Samurai, Giants and the Jaya Sky People who in turn shared their technology with them.

Maybe through exchange, the Giants gift bean seeds to the Sky People which grows from Jaya in the span of the Void Century.

Then tragedy happens in the same century where the Ancient Kingdom falls, instruct its allies to hide poneglyphs for future generations, meanwhile the Sky People split where Birkans and Skypieans flee to the skies through Giant Jack while Shandians stay below and cut Giant Jack to defend the AK’s ideals. Mousse says the ancestors’ souls who died return to the trees.

Calgara says the sound of the Bell is a message to the ancestors who died and went to heaven, he’s talking about the people who fought and died but this might him unknowingly talking about the others that went up in the sky, remember the entrance to Skypiea is called “Heaven’s Gate”.

The Bell tells the ancestors and the world “We’re right here !” for the warriors to return to the sacred trees, for the world to know and for their kin up in the sky.

Uranus’ origins and purpose:

In Greek mythology, among the children of Uranus are the Giants.

Uranus represents the Sky but, just like Pluton was taken from Water 7 to Wano, could’ve been taken from Jaya to Elbaf which has been given the final red Poneglyph.

There’s another theory about Roger stealing a sheet from Big Mom rather than making a rubbing himself just like how Kid says he stole a sheet from Snack and the FI Poneglyph being under Big Mom’s possession while the truly lost red Poneglyph is hidden in Elbaf.

The Sky people in Jaya made Uranus or it -whatever its nature is- comes from them and its purpose, just like Poseidon to the Fishmen, is the bring the Sky people back from the sky to the Blue Sea.

When asked about his intentions, Enel reveals a legend in his homeland about a “Happy return” to a land called “Endless Vearth” where “God” resides.

Enel understands it as the Moon but what if it was a misunderstanding? He only finds the Moon city by chance and, more importantly, Vearth means the earth/dirt in Jaya rather than the Moon.

The happy return is a return to the “Blue planet” with all its Vearth like Jaya and other islands/continents everywhere, remember that “land where God resides” and “endless Vearth” could also refer to the Redline and Lunarians with Whitebeard talking about a “land of gods” atop the Redline, but i don’t know where they fit in here so I’m leaving them out.

Things worth mentioning:

– We have yet to see Urouge, the captain of the “Fallen Monk Pirates” who looked like a Skypiean as a kid and having Birkan wings as an adult (possibly hinting at the shared ancestry between the 3 sub-races being like Fishmen and Merfolk) and could be the Birkan Sky Knight Enel defeated. He also will most likely appear in Elbaf and might go against Kid as a callback to Sabaody but more importantly to see the connection between Sky people and Giants. (On a side note: Kid was about to fight Apoo in the same chapter).

– The Sky Island Birka is South East of Skypiea and also in the Grandline which could or could not be the island that hosted the Poneglyph pointing towards Uranus in Elbaf, we might never know since it was destroyed but, if Urouge is in fact Birkan, could be another incentive for the Mad Monk to reach Elbaf.

– When Dorry and Broggy launch their Hakoku, they say the only thing they can’t pierce is the Bloody Snake/Soaked in Blood, this has always been speculated to refer to the Redline but it also tells us that there might be some knowledge, especially since this a reference to Jörgumandr :”When it releases its tail, Ragnarök (the final battle of the world) will begin.” and the final war after the destruction of the Redline.

– The Wikipedia page about Uranus has this line: Most linguists trace the etymology of the name Οὐρανός to […] Sanskrit varṣá ‘rain’, […] making Ouranos the “rain-maker”, or the “lord of rain” with varṣá being pronounced Varsha and being close to Vasu/Vearth. This could mean anything or nothing but it’s a cool tidbit.

– For a sky Island to float in the sky an Imperial Cumulus is needed, the cloud turn day into night so imagine it can be for Jaya and Shandora (remember Chandra) the night that is brought literally by the cloud where the Sky people flee to and figuratively by the enemy in the Void Century, something like this. Hence them also waiting for the “Dawn of the World”.

– Aside from the Jaya Poneglyph, Robin finds the ancient script on a wall and reads the line:

“Keep thy motives in heart, with closed mouth. We are those who will weave history with the ringing of the great belfry.” (this is not a poneglyph btw, the anime mistook it for one and whoever did the digital color based it on the anime. Just read how Robin says it’s like the script found in poneglyphs and remember the the poneglyphs have a cubic shape + they’re indestructible unlike the wall that shows cracks)

– Check the vol.66 cover where the first two chapters (647-648) are where Sea Kings stop Noah, reveal to Shirahoshi she’s their queen, talk about the promised ship/time and where Fukaboshi talks to his late mother with the “path towards the sun”, Shirahoshi is making the promise sign she did with her family, which is also a pinky promise that Luffy seems to look at while having his back ti Jinbe signifying the friendship and the promise Joyboy(Luffy) will fulfill with Poseidon(Shirahoshi), the same first chapter in the volume has the cover story showing Shandians and Skypieans brought together like in the past.

– Zunesha, Joyboy’s companion, is also carrying AK allies on its back in the millennium old city, which fits with the Poseidon’s and Uranus’ purpose.

Conclusion:

The 3 Ancient Weapons have a specific task which is to bring 2 races to the surface world, Pluton destroys the Redline and FI (while possibly creating All Blue) in the process, Poseidon commands the Sea Kings to carry the Fishmen to the surface in Noah while leaving a “flooded” FI behind and Uranus somehow brings the Sky people down to the surface.

Giants are behind the beanstalk Giant Jack which was given to Jaya, in the Void Century Skypieans and Birkans fled to the sky while Shandians stayed to defend the Poneglyph and Giant Jack was cut in the process, leaving only the top we see in the sky while the root was destroyed along with Shandora.

Giants were entrusted with the task of guarding Uranus, which originates from the Sky people, and the final red Poneglyph for when Joyboy returns.

Theory by Nanto_Suichoken (https://www.reddit.com/user/Nanto_Suichoken)