SPOILERS BELOW FOR THE GREAT MODRON MARCH, DEAD GODS and DIE, VECNA, DIE!
Yeah, that's the short version of what went on in the background in The Great Modron March and Dead Gods.
Tenebrous killed Primus with the Final Word and assumed his position as the leader of the modrons. Then he sent out the Modron March early to find the Rod of Orcus, which had been hidden by servants of Kiaransalee after her takeover of Thanatos.
In addition to Primus, Tenebrous bumped off Maanzecorian, the illithid god of magic, Bwimb, the Baron of Paraelemental Ooze, Tomeri, a goddess of wisdom and love from some Prime world, and the Aztec deity Camaxtli.
Once the March returned to Mechanus and Tenebrous got the information he needed, he scooted off to retrieve his stuff and to return to life. The old Primus became a vestige, and a new one was promoted to replace him. The new Primus saw the anarchy and chaos that the modrons had been corrupted by and recalled all modrons to Regulus, which he sealed off.
Tenebrous succeeded in his plot to return to life as Orcus with the help of the half-ogre priest Quah-Namog. However, while Orcus was resurrected, Tenebrous became a vestige.
Kiaransalee fled Thanatos pretty fast after the news got around that Orcus was back.
Interestingly, while Tenebrous went around killing gods with the Last Word, I think Vecna escaped Ravenloft in Die, Vecna, Die! with the First Word...