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.