A long time ago, there a greater Tanar'ri lord who ascended through the ranks of the Tanar'ri until, after ruling several layers of the Abyss, he eventually attained godhood. Orcus become an Intermediate Power of Death, Undeath, and Vengeance. Unfortunately for Orcus, he was still a Tanar'ri at heart, and he eventually grew old, fat, and complacent, and thus became the subject of some early-edition TSR module that I've never seen, wherein he was pushed from power. Perhaps unbeknownst to the characters of that module, the *real* entity doing the pushing was a lesser god of the drow by the name of Kiaransalee. Orcus had become a dead God.
...much time passes...
Then, a Very Strange Thing happened -- the dead god Orcus began to stir. This was the partial success of The Chosen. Each acting on a different plane (Malazar on the Ethereal, Quag on the Astral, and Madreus on the Prime), they were attempting to use the Uberportals (I never gave them another name, but the Vast Gate inside Firestorm Peak was one, as was Leicester's Gap in the Ethereal -- you would have found the third in the Astral if you had pursued Slade's story back to the Githyanki) to contact Orcus, wherever he might be. The Chosen had discovered some of the mechanics of the Other, and were feeding Incarnations to the `spirit' of Orcus, in an attempt to revive him.
Because you stopped Madreus before he could finish his ritual (he had to commune with the spirit through the gate every day for a month; I think that you guys had something like 4 days left), they weren't able to completely raise Orcus -- instead what they ended up with was roughly an Undead God -- a shadow of his former self, more powerful than a mere mortal but no longer in the same league as he'd once been. Before he could restore himself to his previous power, he'd have to recover his Wand, which still held most of his deific power.
Weakened as he was, he could no longer afford the displays of power and wrath he'd once used. Instead, he adopted a more stealthy, sneaky approach. Given a `new life' and a `new outlook', he took a new name as well: ``Tenebrous'', and set out to recover his wand. Tenebrous began a long, slow campaign to gather information about the multiverse, so that he might be able to locate his Wand.