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.
History
Uranus was a weapon known to have existed during the Void Century. Its actions during this point in history, its specifications, abilities, location, and what became of it since remains unknown.
Trivia
In Greek mythology, Uranus (Latinized from Ancient Greek: Οὐρανός (Ourănós), lit. "Sky/Heaven") was the primordial personification of the sky, the father of the twelve Titans, and the grandfather of the first six Olympian gods. This coincides with the other two Ancient Weapons also being named aftergods of ancient Hellenistic religion.
Vegapunk and Ivankov believe that an Ancient Weapon powered by the Mother Flame was used to destroy the Lulusia Kingdom.[2] It is unknown if this was Uranus since its function and location have not been revealed, however it is the only Ancient Weapon that is currently unaccounted for.[3]
References
↑ 1.01.1One 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.