Upon dying, a wraith might take some initial comfort from the idea that things can't get worse. A Harrowing quickly puts an end to that belief. Harrowings are surreal passion plays intimately tied to a wraith's sense of identity, they take place in the Labyrinth, are set in the wraith's memories, acted out by Spectres and directed by the Shadow, all to the ultimate goal of disconnecting the wraith from the living world and eventually his own sense of self.
Harrowings occur when the wraith is threatened - this can be from physical damage (Corpus to zero) or damage to a Passion or Fetter (any situation which threatens to reduce the rating, except for resolution). When the threat occurs, a nihil opens up under the wraith, sucking him into the Labyrinth. Harrowings can also occur under circumstances that would have caused the wraith (while alive) to die or go mad - such as being crushed under a giant block of soulsteel or seeing the gardens of Yu Huang.
Wraiths are subject to two types of harrowings: the overwhelming majority are target harrowings, where the shadow attempts to weaken the wraith by destroying Fetters, Passions, or the wraith's corpus. In a target harrowing, the quarry is damaged if the wraith fails the harrowing. The other type, destruction harrowings, are reserved for wraiths who are an inch from Oblivion - when they lose the last dot of their last Passion, or all of their permanent corpus; failing a destruction harrowing results in the wraith's destruction or possible conversion into a spectre.
During the harrowing, the wraith is placed in a scenario constructed by that wraith's shadow. The exact composition of the scenario changes with each harrowing, but every one is fundamentally a test of the wraith's character, and it is in some way related to the quarry in question. The choices a wraith makes during the harrowing determine if he succeeds or fails.
After a harrowing finishes, and the wraith survives, they are immediately ejected from the Labyrinth and deposited near one of their fetters.
Consider the wraith of a graduate student, one of whose fetters is an unpublished manuscript, this wraith also has a passion Be Remembered Through Work (Pride).
A harrowing where the manuscript is the quarry might involve something as mundane as the student trying to recover the manuscript once it was lost (with the University becoming more threatening and surreal as he hunted for the work), or something more exotic and directly threatening, like being accused of plagiarism and the work being the central evidence for the accusation. An alternative lesson might involve learning to accept one's death, in which case the wraith of might have to accept that the manuscript will never be published - in which case, saving the manuscript may actually fail the harrowing.
Metaphysics of Harrowing
The harrowing was originally a form of "tough love" in the Underworld - intended to help wraiths come to terms with their own deaths when they weren't willing to come to terms with it themselves. The Underworld has been corrupted by Oblivion since that time, but the evidence of Harrowing's original purpose is obvious to those who look for it. Most notably, spectres have relatively little control over the scenario - they cannot harm the wraith outside of the rules of the harrowing, can only get involved as actors, and must ensure that there is a way to resolve it, that it is not inescapable. Spectral theologians debate the origins of the rules of harrowing, and why they are inviolate, with as yet no definitive answer.
The existence of harrowings has a variety of side effects on wraithly society; any wraith can opt to dive headfirst into the ground and trigger a harrowing to get away from pursuers. The presence of this ultimate "get out of jail free" card has led to a variety of techniques for restraining wraiths who don't want to be held down.