The Harrowing is a test that every mage apprentice must go through to become a full Circle mage. The test involves a mage entering the Fade with the use of lyrium. Once in the Fade, the mage apprentice must face, and subsequently overcome, a demon who wishes to posses the mage's body and enter the living realm. The demon is summoned by the mages of the Circle of Magi to the same part of the Fade the mage apprentice enters and is promised a living body will be waiting for it when it arrives. If the mage apprentice succeeds in resisting the demon, he is granted the title of mage and becomes a full member of the Circle of Magi.
However, if the mage is overwhelmed by the demon, or takes too long to complete his task, the mage is killed by a group of templars who are standing by so the mage does not become an abomination and is not a threat to the Circle Tower or the rest of Thedas.
Because the Harrowing ritual requires a significant amount of lyrium, only promising mages are deemed worthy of attempting it. Those who are not are either forced to become one of the Tranquil or are executed. A mage can also elect to become one of the Tranquil if they wish. Since passing the Harrowing theoretically proves that a mage is capable enough to resist possession, Chantry law forbids making mages who pass their Harrowing Tranquil.