Before i start this theory i would like to brief you on what we currently know about Uranus:
- We know that the ancient weapon was created in the void century.
- We know Uranus has something to do with the sky.
- We know Uranus is capable of destroying the world.
But what is it?
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Many people speculate that Uranus was used to create devil fruits in the tree of the devil (look this theory up yourselves, there are many btw).
Uranus relates to the sky and the heavens. In Enels paintings there were three Ancient Weapons that could be seen the sun, a ship and the sea. Ship and sea sound familiar? 
Three people on a ship, one wears a fish hat, one has a ship looking hat and one has antennas, the ship they stand on is… pluton. Three other people on a ship, under that ship are sea-kings, ill let you guess what weapon that is. Above everything else is the sun (Uranus)
The person highlighted with the antennas holds a staff, remember that for later.
So here is the theory, Uranus is a staff made from seastone that can absorbs the souls of creatures. This was technology originally developed by the moon people. When the moon people saw the humans that well on earth they decided to show their appreciation and kindness and shared their knowledge with the Ancient Kingdom. When the ancients learned of such technology they used it for the humans betterment, that’s why moon people don’t come down anymore coz humans did them dirty.
Try and keep up for this next bit.
Uranus is a staff created from seastone that can contain and transfer “souls”. In the Void Century many races co-existed in the world of One Piece, when the humans learned about such technology from the moon people (the ancestors of the Skypeians, Birkans and Shandians) they used it, as humans would, for their own good, hence the creation of the primitive World Government. Poseidon was used to attack the races underwater, Pluton was used to attack the islands and Uranus was used to enslave the slaved races by stealing their souls. Obviously some good humans were against it (a lot actually) and therefore started the war.
With their new gained souls they were manifested into a tree of all devil fruits in Mariejois (that’s the national treasure) so it was consumable by humans, the humans then gained the upper hand and then the other races became extinct. The sun is a giver of life and is responsible for all life on earth so that’s why the sun represents Uranus along with the sky theme. The guy with the antennas is the race that knew the tech behind Uranus since he holds the staff.
This is why df users can’t swim, not because the soul wants to go to the sea but the opposite of what was thought, the sea resembles the seastone used to create the staff of Uranus hence the weakness to seastone and sea due to the souls wanting to return to Uranus. Shanks said the df has the soul of the seadevil, just one of the unknown races whose souls were used to create dfs. I believe Kaido could be a seadevil, like an actual one due to the fact they still can exist probs. Note other exotic species
Admiral Shiroryu learned of this and took Uranus from World Government, Monkey D. Dragon or “Admiral White Dragon” took the weapon and became a Revolutionary (Garp never knew this coz he probs never cared and was too busy as a VICE ADMIRAL fighting Roger and stuff). And so he has that currently which is why he can be seen as such a op threat to WG.
This would also explain how Marshall D. Teach knows about consuming more than one df.
So Blackbeard is also from the race of the moon people?