I have such amazing things to show you
A lifetime's worth of mistakes and fears
Every little error you've made
These puppets are mine to control

The Book of Truth and Joy
- Anon.

A Harrowing is a hideous psychodrama that plays out inside the Wraith's senses, where the Shadow tries to assert dominance upon on the Wraith: showing her that it is right, and she is wrong. The Wraith must find some way to persevere through this, or else lose to the Shadow, who becomes that much stronger for its dark victory.
To begin the Harrowing, the Shadow must spend almost all the Tainted Essence it has, taking itself down to one. However, note that in the case of a failed roll to stave off mental imbalances, following a failed Degeneration roll, a Harrowing occurs without the Shadow having to spend anything: see Morality for more information.

When this happens, the Wraith is immediately plunged into a catatonic state, and "wakes up" in the bizarre, nightmare reality the Shadow has created for her. The Shadow then brings to life an odd and twisted morality play, giving the Wraith a choice to embrace the Shadow's point of view, or else sucumb to its power.
In game terms, at the climax of the Harrowing, the Storyteller rolls the Wraith's Presence or Manipulation - whichever is higher, or more appropriate - plus the Shadow's Angst, to repesent the Shadow's side of the matter. The Wraith's player then rolls her character's Manipulation or Composure - whichever is higher, or more appropriate - plus the Wraith's Eidolon. No Modifiers are allowed on this roll.
A failure on the part of the Wraith means that she chooses poorly, or submits. A failure by the Shadow means that the Wraith saw past the lies, or stood firm. Either way, the Harrowing ends, and the Wraith is brought back to her senses - however befuddled they may be - either better or worse for having gone through the ordeal. Meanwhile, the Shadow is exhausted by the effort, and cannot say or do anything for a day and a night.

If a Harrowing should result in a Shadow's Angst going up to 10, then the Wraith immediately becomes one of the Damned. The connections to her Anchors are cut, and she falls through the Barrier, screaming with the joy and relief of the Shadow. The character is no longer a suitable player character, and the ST should take possession of the sheet.

Cost: All the Tainted Essence in the Shadow's pool, minus one
Dice Pool:  Presence/Manipulation + Angst vs. Manipulation/Composure + Eidolon}
Action:  Contested
Dramatic Failure: N/A
If the Shadow fails, the Wraith succeeds, and leaves the Harrowing feeling vindicated, regaining a dot of Willpower for her trouble. If she was Lost, she is no longer so.

If the Wraith fails, the Shadow succeeds and gets another point of Angst, and if the Wraith was Lost, she remains so. In addition, if the gaining of another point of Angst should overlap the Wraith's current Morality, then that dot of Morality is lost, forever.
Exceptional Success:
An exceptional success for the Shadow not only gets ita point of Angst, but gives back one point of Tainted Essence per success past the first. Wraiths, meanwhile, not only come "back" if they were Lost, but also regain two dots of Willpower due to the extreme self-affirmation of the experience.


Diceless Options and Inspirations

Note that the roll to resolve the Harrowing is not entirely necessary: Storytellers are encouraged to take this matter beyond dice, and allow the day to be carried by good roleplaying on the part of the Wraith. They can simply judge the matter to be a success or failure for the Wraith based on how the player chooses, and why.
More details on Harrowings, what you can do with them, and how to pull them off, can be found in: Wraith: the Oblivion 2nd ed, Wraith Player's Guide, Shadow Player's Guide and The Sea of Shadows.

For the purposes of  Wraith: the Arising, don't pay any attention to the rules for Target Harrowings or Destruction Harrowings: these distinctions are obsolete in the WOD 2.0 setting.