here is a place between dimensions, between worlds. To some it is known as the Outside, to others it is the Unknown. To those that truly know, however, it is called the Void. Despite its name, the Void is not an empty place, but rather the home of unspeakable things - horrors not meant for minds of men. Cho'Gath is a creature born of the Void, a thing whose true nature is so awful most will not speak its name. Its fellows have been poking at the walls that divide dimensions for a crack, a way into Runeterra, where they can visit their own personal paradise of horror upon the world. They are called the Voidborn, creatures so ancient and terrible that they have been removed from history altogether. It is rumored that the Voidborn command vast armies of unspeakable creatures on other worlds, that they were once driven from Runeterra by powerful magic lost to antiquity.
Source: The Game Website.