Priests cannot convert aligned altars, because they respect those other religions. However, they can convert Moloch’s altars, including the ones in Gehennom.

A new type of trap called a “conduct trap” that spawns in Gehennom. Triggering one causes Moloch to enforce one of the in-game conducts on you for the rest of the game, or until you next break it, even if you have previously broken it. Breaking it after this happens makes Moloch mete out some sort of harsh punishment.

Dropping items onto a Moloch-aligned altar (or only his high altar?) can curse them.

Assuming an implementation in which Gehennom starts to get destroyed after you remove the Amulet: When you kill the high priest of Moloch, he gives you a multiline death message calling you a fool and saying you don’t know what you have done, and maybe hinting a little bit at why Hell is about to go to hell.

Replace all the priests of Moloch on the Astral Plane with (still hostile) priests of the other gods. Moloch is a disgraced, hated god, and his priests have no business there.

When you kill the high priest of Moloch, Moloch himself shows up with the Amulet, and you have to beat him up and take it from him.

One Gehennom “special room” should be an active temple with a peaceful priest of Moloch.

Aligned altars can generate in Gehennom, and standing on a coaligned altar in Gehennom is the only way to contact your god (via prayer or sacrifice). If the altar is crossaligned, you can try to convert it, but Moloch may seize control during the process and turn it into an altar to Moloch. Molochian altars in Gehennom are very difficult or outright impossible to convert.

Wandering hostile priests of Moloch occasionally generate in Gehennom.

You can invoke the Amulet on Moloch’s high altar to destroy it; this does something good later for you (more points? prestige?) but Moloch will now harrow and hound you down all the way back through Gehennom even more fiercely than usually.