The Hierarchy is the government of Stygia in Wraith: The Oblivion. It rules over the Western dead, specifically Europe, North America, and to a certain extent countries colonized by them (such as Australia).
The Hierarchy primarily operates through the eight Legions, which souls are alloted to based upon their mode of death. In addition to these Legions, wraiths may belong to one or more Guilds, some of them sub rosa, some official. The exact governing structure of the Hierarchy is in flux; it was originally designed to operate under the auspices of Charon, whose disappearance in 1945 during his fight with Gorool has thrown the entire organization into flux. Hierarchical society is just as driven by the weight of massive tradition as by legal status. This is at least partly because the Hierarchy's continued existence is something of a feedback loop: older wraiths transfer passions to maintaining the Hierarchy, and the Hierarchy exists to provide older wraiths a place for those passions. The closest thing the Hierarchy has to official leaders are the Deathlords, the leaders of the seven Legions who were chosen by Charon.
Following the events in Ends of Empire, the Hierarchy is effectively destroyed.