The Reaper was a strange being who seemed to rule an interplanar crossroads that took the form of a room with seven doors, which the Reaper called the Gatehouse. His realm was accessible to whoever held an item called the Relic of the Reaper.
In 1372 DR, the Relic came into the possession of Drogan Droganson's Pupil, whilst they were briefly trapped in the Plane of Shadow after the second fall of Undrentide. The Relic of the Reaper used items known as rogue stones to provide teleportation services for the hero, allowing them to set up a network of portals so they could mark places and return there by passing through the Gatehouse. The Reaper was also able to snatch the hero from the jaws of death by using a rogue stone to bring them to the Gatehouse instead of the Fugue Plane at the moment of death.
Drogan Droganson's Pupil eventually was slain by Mephistopheles in the Underdark, at which point the Reaper revealed that the Relic was a piece of the archdevil's own flesh. The hero was now consigned to Cania as Mephistopheles normally was, while the fiend himself was free to wreak havoc on Toril. The Reaper explained that Mephistopheles had bound him using the power of his true name and that only someone who knew that same true name could command him to open the Gatehouse to stop the archfiend. The Reaper heavily implied that his service to Mephistopheles was forced and that he would be happy to be free. When the hero successfully learned the Reaper's true name, Hecugoth the Abandoned, the Reaper admitted that he liked serving the hero, but that they would never meet again. He then allowed the hero and their companions to return to Waterdeep to confront Mephistopheles.