In NetHack, the Invocation is the little ritual you have to perform, using the Invocation Artifacts, to get at Moloch's Sanctum and acquire the coveted Amulet of Yendor. Full disclosure follows -- if you don't want to hear it, back out now!

On the very bottom level of Gehennom, there is a square that vibrates when you stand on it. Make sure you have all of the Invocation Artifacts, and that they are all uncursed or blessed (it's alright if the candles for the Candelabrum are cursed, though). Take a deep breath, and then...

  1. Install the seven candles in the Candelabrum
  2. Light the candles
  3. Ring the Bell of Opening
  4. Read the Book of the Dead
In game terms, this means apply the candles to the Candelabrum, apply the Candelabrum, apply the Bell, then read the Book. If you do all this correctly, a staircase down and a moat should appear. Once this happens, you can discard the Invocation Artifacts as they are no longer necessary. You can then proceed downstairs to the real bottom of Gehennom, where the Amulet of Yendor is held by the High Priest of Moloch.