Uranus is one of the three Ancient Weapons that existed during the Void Century along with Poseidon and Pluton 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'.
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. Uranus was a weapon known to have existed during the Void Century. Its actions during this point in history and what became of it since currently remains unknown. Like Poseidon, it was named after a Greek deity. Uranus (Ουρανός) was the personification of the sky, the father of the twelve Titans, and the grandfather of the first six Olympian gods. "Ouranos" can literally translate to "Sky" or "Heaven" in English.