The Passage is an ancient Necromancy event that was to be set in motion by the Death Bringer in order to stop death itself. The process was a mystery to all except those in the highest places of power in the Necromancer order.
It is later revealed that half the world's population needed to be killed for the Passage to occur. The Necromancer belief was that life and death flowed into one another in a "stream." The Death Bringer was needed to dam this stream.
Importance in Books
Valkyrie later uses this knowledge to taunt Melancholia St Clair, making fun of the fact that Valkyrie, a recent Necromancer, already knew more about the Necromancers' religion than Melancholia, who had grown up in the Order.
Vandameer Craven, tired of being ridiculed, uses his knowledge of the language of magic to carve signs into the skin of Melancholia, looping her Surge and boosting her power. While her looping power meant that she would feel ebbs and flows, Melancholia is hailed as the Death Bringer by the Necromancers, who calculate that she would eventually have enough power to begin the Passage.
Valkyrie finds out the number of lives needed for the Passage from Wreath and is mortified. She passes on the information to Skulduggery, and they both agree that Melancholia must be stopped. The plan for Melancholia to usher in the Passage is later abandoned, Melancholia deciding she would rather kill the entire population of the planet.
It is implied that the secret massacre required for the Passage became widely known. Wreath attempted to convince Cleric Craven to give up his plan with Melancholia and claim that the belief belonged to only their Temple in order to save other Necromancer Temples from persecution around the globe. Craven refuses. With no options left, Skulduggery is forced to turn into Lord Vile to stop Melancholia, and she is knocked out during the battle between Vile and Darquesse. Later books seem to show Necromancers assimilating to life outside Temples.
The Passage was described as the event to bring about world peace. Wreath explains that without the threat of death, war would stop, and a new era of enlightenment would begin. Of course, the price for this world peace is huge; before the events of Death Bringer, only those in command at Necromancer Temples knew the exact process of the Passage.
Following the Necromancer belief that life and death flow into one another, the Passage was to stop this exchange of life. Scientifically, it is explained that the shock of so many souls dying at once would "dam" the river of life and death. No new children would be born, and all those alive after the initial massacre would stay alive forever.