Xoriat

Xoriat, called the Realm of Madness, is a plane that defies description. By merely visiting the plane, non-natives risk having their minds shattered. Over 9,000 years ago, Xoriat became coterminous with Eberron, and the continent of Khorvaire was overrun by the daelkyr and their aberrations (known as the Daelkyr War). The daelkyr destroyed the Dhakaani Empire, but were pushed back to Xoriat by the Gatekeeper Druids. Dimensional seals created by the magic of the Gatekeepers have disturbed Xoriat's orbit, preventing it from becoming coterminous again. Since then, the Gatekeeper druids patrol the seals to Xoriat, making sure that no abominations return. However, not all daelkyr were destroyed; at least six of them remain on the Material Plane, hidden and waiting, presumably in Khyber.
Some theorize that the Madwood forest of Aerenal is a Manifest Zone of Xoriat, while others think it is simply cursed. There is a small manifest zone to Xoriat in the The Nightwood in Karrnath. There are also numerous manifest zones to Xoriat in the Shadow Marches. However, the Gatekeeper druids placed dimensional seals at these planar intersections and even thousands of years later, they remain unbreeched.