OK, I will try...
SPOILERS for "The Great Modron March" and "Dead Gods!"
Once upon a time, a drow deity of undeath - Kiaransalee - overthrew Orcus and stole his realm and slew him.
A long time later, Orcus awoke again - as an undead deity. He slowly gathered strength again, and on the third layer of Olympus, he found the secret of the Last Word, a word that could slay even deities if spoken. But to fully regain his power, he needed his wand, which contains much of his power. And he has no idea where it is. All he knows is that Kiaransalee had it at one point.
Kiaransalee gave it two living drow worshippers to bury it on the deepest layer of Pandemonium, then promptly drowned them in the River Styx to make sure they won't remember the location even if they come back to life.
To get a lead, Orcus (who now calls himself Tenebrous) confronts Primus, the ruler of the Modrons. When Primus truthfully tells him that he has no idea where the wand is, Tenebrous slays him and takes his place. Then he triggers the Great Modron March, a "swarming" of the Modrons across all the Outer Planes which normally takes place every 17 times 17 years - and isn't scheduled for another 100 years or so. Naturally, this causes some Comment, and the repercussions are explored in the adventure of the same name.
At the end, Tenebrous discovers a few leads to the whereabouts of one of the drow, and sends his minions out to follow them. In "Dead Gods", the PCs gradually learn what is at stake, and race to get to the wand first - and hopefully dispose of it...
If they are successful, then Orcus is dead - again. But the last adventure brings to mind that famous Lovecraft quote:
"That is not dead which can eternally lie..."
