A grotesquely fat, ram-headed, bat-winged demon, Orcus ruled over a court of tanar'ri and the undead, hating both with equal vehemence. The greatest measure of his spite, however, went to his rivals Demogorgon and Graz'zt; each of the three coveted the whole of the Abyss, and were unwilling to share any part of it.
For millennia they warred, until Demogorgon's civil war with his other self began to distract him from the battle. At the same time, Graz'zt was imprisoned on the Prime+Material by a wizard of great power, the witch-queen Iggwilv. With his greatest rivals hamstrung, Orcus grew complacent and lazy, letting his defenses slacken. Who would dare attack the greatest remaining lord of the Abyss?
Enter Kiaransalee, an ancient goddess of undeath and vengeance. For the sake of some offense that had been forgotten by everyone but her, Kiaransalee invaded Orcus' layer of the Abyss, Thanatos, and slew its lord, casting a mighty spell that erased his name from every record. Taking the icy layer of Thanatos for herself, she butchered or exiled those of Orcus' servants who would not serve her, and settled in.
For centuries Orcus drifted, a forgotten corpse in the Astral Plane.
A few years ago Orcus returned as a shadowy undead being calling itself Tenebrous and walked the planes in secret, wresting the power of the Last Word and slaying many gods and immortals to find a relic of his power that Kiaransalee had hidden. Though the Last Word consumed him, his last worshiper used the wand in a ritual designed to resurrect him fully; though the ritual failed, still Orcus managed to return.
Now Orcus rules Thanatos again, with new energy and cunning directing his rage more effectively than ever before. The aspect of him that was called Tenebrous, however, the part that was truly divine despite its greatly diminished power, was separated from Orcus the Tanar'ri, becoming a Vestige.