The Thirdspace Invasion occurred after the Second Shadow War and before Sheridan's Campaign to liberate Earth. It began with the discovery of a Vorlon-constructed artifact that led into another universe called Thirdspace. This alternate dimension was inhabited by a race of hostile, very powerful telepathic aliens that had attempted to invade our universe thousands of years before. Though the Vorlons had repelled the invaders, the aliens enslaved several Vorlons and used them to fling the portal into hyperspace. It was rediscovered in 2261 by a team of humans, who then brought it to Babylon 5 study. The Thirdspace aliens became aware of this and planned again to invade our universe and destroy all life.