A semi-permanent portal between Tamriel and the Slipstream Realm; a series of trans-liminal points between the Prime Material and other planes of existence. At one point in time, the Tamriel Empire built an academy known as The Battlespire within the Splipstream Realm.
According to history, the Weir Gate has officially been sealed by Daedric forces during the time of the Imperial Simulacrum.