Sanctum of the Fallen God is located inside the Temple of Atal'Hakkar. It is located through the western tunnel from the Pit of Sacrifice and can be reached without killing any particular mob in the dungeon. Sanctum of the Fallen God is the location of Avatar of Hakkar, which is only killable by using an [Egg of Hakkar] or [Yeh'kinya's Scroll], both of which are obtainable from Yeh'kinya after a certain point in his quest line.