The Apocalypse Stone – Starring Moloch!
OK, I finally got around to looking it up. This module by Jason Carl and Chris Pramas was from 2000, and therefore takes place after Guide to Hell came out and was one of the last products released before the debut of 3rd edition (not un-coincidentally). The part involving Moloch starts on page 47. It begins by briefly recounting how he got into trouble into the first place, and that he plotted his return for centuries, by marshalling resources, winning allies, and preparing a surprise attack to take back Malbolge. He went to a Material Plane world to hide and finish his plan, preparing to plane shift back to Baator to begin his invasion. However, he had the misfortune of choosing the world on which the module takes place, which gets cut off from the Outer Planes during the course of the module, stranding him on that world and allowing his armies to walk into a slaughter at the hands of the Hag Countess’ forces. Furious, and mad for revenge, he sets out to find and destroy those responsible for his situation – the PCs, of course. Pages 48-54 detail Moloch’s machinations against the culprits, culminating in a deadly confrontation. He does not appear in the rest of the module, since it’s assumed that when he confronts the PCs, either they kill him or he kills them.
Even if he does kill the whole party, a DM might assume that he simply dies when that world ends. However, WotC does not assume this, since he is identified as alive in more than one 3E product – after all, the option is there in the module to actually have the PCs save the world (though the designers clearly have more malicious havoc in mind than that contrived ending!), so there is that. One must assume that, to place him properly in canon, he must have found a way to survive and return to Baator, where he was quickly returned to an outcast status on Avernus to plan his revenge anew.