Mardum, the Shattered Abyss

Mardum, the Shattered Abyss, formerly known as the Plane of Banishment; colloquially Mardum is a now-shattered planet that exists throughout the Twisting Nether akin to Outland that was initially created by the Titan Sargeras to imprison all demons he'd capture, however, when he fell into madness and became a Dark Titan, Sargeras shattered the Plane to release all the imprisoned demons who would become the first demonic soldiers of the Burning Legion.

The relic known as the Sargerite Keystone capable of opening portals to many demon worlds was kept guarded on this planet. In order to access these demon worlds as part of his plans to defeat the Legion, Illidan Stormrage sent his Illidari Demon Hunters to Mardum during the time of the Sha'tar's siege on the Black Temple near the end of the Burning Crusade era to retrieve the Keystone from Mardum's new ruler, Brood Queen Tyranna.