The Harrowing: Survive the Harrowing
Before beginning the walkthrough, it's probably a good idea to make sure that you don't take healing spells. Instead, settle on offensive ones. The reason why will become apparent while playing.
After the cinematic, a conversation will occur. Go through the dialog and you'll be transported to the Fade. Upon gaining control, open the journal. Read it and right click the Disturbing Statue for a codex entry. This will also give some experience points. Also, loot the vase.
Afterwards, take the path leading south-west on the mini-map. A Wisp Wraith will appear. Defeat it and the two that appear further on. After the third wisp, continue. A dialog sequence will occur. It'll become apparent that not everything is as it seems in the Fade. The mouse will follow you, but be uncontrollable. Then continue on the path.
You'll be in a circle of fire. Right click the Lyrium Vein, then take the path leading south-west. Kill the Wisp Wraiths and before talking to the Spirit of Valor, loot the vase and click on the Twisted Structure. Talk to the spirit about the weapons. If your willpower statistic is high enough (which it should be), you should be able to acquire the spirit's weapons without a duel. Then continue.
Defeat the Spirit Wolves when they appear. Continue along the path. Talk to the Sloth Demon. You can either fight the Sloth Demon or have it teach you the bear form by answering riddles. Answer the riddles. The correct answer for the first riddle is 'a map'. The second 'a tongue'. The third 'a dream'. After passing the test, Mouse will become a bear and a controllable party member. Now head back.
At the Spirit of Valor, more Spirit Wolves will appear. Kill them. Continue to the circle of fire. Don't click the Lyrium Vein yet. Talk to the demon that appears. After the dialog, kill the demon. It's not too difficult and it's just a matter of using Mouse's abilities. Do note that fire spells won't damage the demon though. Also, Wisps will spawn. If playing on normal, ignore the Wisps. After it's defeated, another dialog will occur. Choose the 'I'm starting to think the other demon wasn't my test' option when it appears. After the dialog, this part is complete.