It was first mentioned during Nico Robin's conversation with King Neptune, when she says there are three ancient weapons that bear the names of 'gods'.[1]
Like Pluton and Poseidon, Uranus is a dangerous weapon which can be used to rule the world or, in the wrong hands, destroy it. But as of yet, absolutely nothing else is known about it.
History
Uranus was a weapon known to have existed during the Void Century, Its actions at this point in history and what became of it since currently remains unknown.
Trivia
Like Poseidon, it was named after a Greek deity. Uranus (Ουρανός) was the personification of sky, the father of all Titans, and the grandfather of the first six Olympian gods. "Ouranos" can literally translate to "Sky" or "Heaven" in English.
References
↑One Piece Manga and Anime — Vol. 66Chapter 650 and Episode 570, Robin (Neptune in the anime) speaks of Uranus along with the other two weapons' names.
