Felon,
if you're still looking for ideas about an orc horde/orcus alliance...
The orcs already have an established Death God in their pantheon: Yurtrus. But perhaps Gruumsh is beginning to sense that Yurtrus does not instill the fear of death that he once did. Perhaps his northern tribes have seen an influx of Tanarukk (demon-orcs from hellgate keep) who whisper of a new lord of death more terrible than Yurtrus.
Perhaps Orcus seeks a place in the orcish pantheon, to draw yet more souls into his abyssal palace. He offers something to Gruumsh that Yurtrus can't... He promises that each orc who is slain gets a second chance to fight.
So a brutal inquisition begins within the orcish tribes. Followers of the old way are driven out, converted, or killed. Few cultures will welcome the exiled orcish death priests, so they are driven deep into the earth. Meanwhile, the Tanarukk shamans of Orcus have already whipped the tribes into a frenzy of bloodlust.
King Obould is the war chieftan chosen to drive these tribes out of the mountains. The horde has been building for some time, so the allied forces of human, elf, and dwarf are well-prepared. Heavy casualties are felt on both sides but the allied forces completely crush the first wave of the horde...
...until Orcus animates each orc warrior at sundown that day. Undead orcs slaughter the allied defenders and ravage the northern countryside.
Gruumsh, revelling in the complete and utter devastation, heaps favors upon Orcus even as the followers of Yurtrus shrink ever deeper into the shadows.
Perhaps, if your heroes are high enough level, this epic story arc could culminate with heroes sneaking a champion of Yurtrus into the Abyss to confront the demon lord on his home turf.
An orc horde should be terrifying to begin with. By adding the unknown mastermind (orcus) and the horror factor of undead, I think you've got the seeds for a really cool, very memorable campaign.
-eric