The Stronghold as seen from without. Art by Jeff Miracola.
The Rathi Overlay was the second stage of the Phyrexian Invasion. It involved the merging of Dominaria with the artificial plane of Rath, transporting all Phyrexian troops on Rath to Dominiara. It was a success, even though it interfered with the Aether, causing many weird mutations among the inhabitants of both planes. That transfer also dropped some creatures into the shadow form (notably people of Rathi Tribes and slivers).
Examples of creatures mutated by the Overlay are the Volvers, Gaea's Skyfolk, Razorfin Hunters and others.