EDIT:
This is what Tome of Magic says on the matter of Orcus and his quest for divinity:
(I hope I can post this here, if not please remove it as admin else Ill remove it in a bit myself.)
The great demon lord Orcus has long sought divinity. Some years ago, for a brief period, he actually attained it. Slain and resurrected by a surge of negative energy, the sorpulent demon arose as the gaunt Tenebrous, a god of darkness and undeath. For a time he traveled the planes in this form, slaying other gods in his quest to gain more power. His ultimate goal was to reincarnate himself yet again because he wished to be a god as orcus, not Tenebrous. Some say he was thwarted by a band of mortal heroes, but whatever the cause, his grand plan failed. Orcus did indeed rise again, but as the demon prince he once was, not a god.
True divinity can never fade completely. The tatters of godly power that Orcus shed remained intact. Less then a god but still divine, this bit of essence drifted in the void between planes until it once more coalesced into a bitter sentience.
Thus, Tenebrous yet exists as a pale reflection of what he once was, a shadow of a shadow.