The Harrowing is the first sub-quest of the Magi Origin story, "In the High Tower of the Mages". It starts immediately after the opening animation, at the end of which you touch the lyrium and go into the Fade.
Enter The Fade Edit
You will start at the top right (northeast) of the map. To your left, you can find a Vase of Healing containing Shimmering Orbs of Light. These can be used to heal yourself whilst in the Fade but will vanish from your inventory once you complete your Harrowing, so don't be afraid to use them. (ps3 and xbox360: instead of Orbs there are Health Poultices in the Vase; if unused these will stay in your inventory after you leave the Fade.) Next to the Vase is a Disturbing Statue which will unlock Codex entry: Beyond the Veil: Spirits and Demons. Progress through the Fade to find a few Wisp wraiths that attack with lightning. Fortunately they come at you one at a time, and shouldn't require much more than an Arcane Bolt to kill.
Along the way, you will meet what appears to be a talking rat, who will introduce himself as Mouse (he may explain that he's been in the Fade too long to remember his real name). Mouse tells you he was once an apprentice like you, but that he was murdered by the Templars for failing to complete his Harrowing. He will explain a bit about the Fade and your test, and warn you about other spirits in the area. Further down the path is an open area to the right. Mouse will explain that this is where the demon you are to face will manifest. The Lyrium Vein seen here (which unlocks Codex entry: Lyrium) can be used to recover Mana during combat (and will replenish itself later if used now).
Spirit of Valor Edit
To prepare further for the battle, proceed westwards. The path will open up again. Up a slope to your right are another vase of orbs and a Twisted Structure (which unlocks Codex entry: The Black City). To the left is Valor, who may reveal during conversation that he can create weapons with his will alone. If you ask him for a weapon, Valor will challenge you to a duel. To obtain Valor's Staff, you can take him up on this challenge and win the duel. (After winning you can attack and kill him for XP.) Alternatively, you may accuse him of plotting to kill you, then demand he prove his good intentions by helping you. Equip your new staff and progress onwards, and fight a few spirit wolves.
At the far west of the area, you will fight 3 Spirit Wolves and [further on] meet a Sloth demon in the shape of a bereskarn. There are three ways he will agree to teach Mouse to take on his form. You can defeat him in combat or answer his riddles; if one riddle is answered incorrectly, you'll have to fight him. The answers to his riddles are, in order, "a map", "my tongue" and "a dream". Alternatively, if previously you put enough points into cunning, you can tell him you would pester him endlessly until he gives in. After you defeat Sloth one way or another, he will teach Mouse to turn into a bear (though not a bereskarn) and then resume his "rest". Mouse will now join the party as a companion. If you've not done so already, you may have more of a discussion about the Fade and Mouse's memories.
The Spirit of Rage Edit
Now your preparations are complete; double back toward the middle of the map. More spirit wolves will attack you - but now you have Mouse in bear form on your side. The Spirit of Rage now appears in the central area you passed earlier. After some dialogue, battle will ensue. Four Wisp Wraiths will join the fight. You can ignore these and focus on the Spirit of Rage, because once it is defeated the battle will cease. However, it is advisable to take them out for easy XP; usually, Mouse will distract the rage demon while you are attacking the wisps.
If you have it, Winter's Grasp is very effective against the fiery Spirit of Rage; conversely Flame Blast won't help you here. If your mana gets low, remember to use the Lyrium Vein, if you haven't already done so. If not, then Mouse in his bear form should be able to occupy the Spirit of Rage while you cast spells or use your staff from afar. Don't forget the healing Shimmering Orbs of Light, which will shortly become useless for anything else, and which Mouse can also use.
Completing the test Edit
Once the Spirit of Rage has been defeated, Mouse will want to discuss a proposal ... but it swiftly becomes clear that Mouse isn't the timid little individual he pretended to be. Once you make clear you see through Mouse's ruse, he will shift into his true form (though this is not clearly shown on screen due to camera angle, its implied size and a fair bit of Mouse's dialogue suggest he is really a Pride Demon). Resisting Mouse, it turns out, was the real test, which you have now passed.
If Mouse dies during the fight against the Spirit of Rage, the dialogue with Mouse still happens but you do not fight the Pride Demon after the cut scene.
Codex entries EditCodex entry: First Enchanter Irving Codex entry: Knight-Commander Greagoir Codex entry: The Harrowing Codex entry: Beyond the Veil: Spirits and Demons Codex entry: Wisp Codex entry: Wolf Codex entry: Sloth Demon Codex entry: Rage Demon Codex entry: Lyrium Codex entry: The Black City
- Depending on what actions are taken, various Controls codex entries