In
NetHack, the Invocation
Artifacts consist of the
Bell of Opening, the
Candelabrum of Invocation, and the
Book of the Dead. In order to acquire the
Amulet of Yendor, you need all three of these
artifacts, plus seven
candles for the
Candelabrum (those are most easily found in
Itzchak's Lighting Shop in the
Gnomish Town, so make sure to grab some while you're there).
Once at the bottom of Gehennom, the Invocation Artifacts are all used together in the Invocation, which opens the way to Moloch's Sanctum. I'm not going to give away the process here, though -- see the description of the Invocation for the spoily information.