Over time, Spencer's Lyrium-caused instability grows steadily worse.  Eventually, it becomes so much that he seeks a solution, any solution, and believes that a journey into The Fade and getting help from a Spirit may solve the deadly addiction. He gathers what friends he has (Naomi, Kalas, and one other, possible Spider-Man.  Four is the limit to Dragon Age parties.  And he probably doesn't have many friends at this point anyways).  To get into The Fade, it is required to find a place where The Veil would be weak.  These places are generally where much magic usage and death took place.  The most obvious choice for this is the Keyblade Graveyard.

In the Keyblade Graveyard, Spencer has to first send his spirit into The Fade, and then 'grab' the other's unconscious minds to drag in with him.  This requires the others to be asleep, which Spencer does with a Sleep spell, and for him to 'open' The Veil, which requires ingesting large amounts of Lyrium (not like it can get worse at this point).  Once inside the Fade, the group begins to seek out a kind Spirit.  Luckily, the Fade also seems to have the effect of allowing Spencer greater self-control, keeping him significantly more stable while inside.

Along the way, each member of the group will be tempted by various Demons.  If they do succumb (which I doubt, but just in case) the other members will be forced to fight the one who is tempted.  The defeated group member will be ejected from The Fade and awaken outside, with no damage done but to their conscience.  Kalas will face Desire, Naom will fight Rage, and Spider-Man will go against Sloth.  They will learn that there are good forms of the sins that these demons represents (like how Justice can be perverted into Vengeance).  Desire is Devotion, Rage is Passion, and Sloth is Patience.

Eventually, they come across a Spirit that agrees to help Spencer.  However, it turns out that it was a Pride Demon, who had preyed on his weakness and overconfidence that he could handle anything in the Fade, due to being an expert.  The Demon begins to possess Spencer, attempting to turn him into an Abomination, but he fights back, realizing that a Demon can only possess a mage if they accept it, and also realizing that he was overconfident about his chances in the Fade because he was more worried about his friends facing the demons.  He manages to absorb the Demon, merging it with his own spirit, much like Anders and Justice, curing his Lyrium addiction.

As the group leaves, however, a plethora of Demons follow them out of The Fade, due to Spencer underestimating how large the entrance.  The group is forced to fight the Demons and give Spencer time to close The Fade, lest more get out and spread across the worlds.  As the fight goes on, Spencer becomes enraged, and because of his merging with the Pride Demon, activates his new form, Honor (based off of the Battlemage specialization from Origins, as well as a spell from Arcane Warrior).  The appearance of the form is similar to Anders' Vengeance, but with the aura that surrounds him and his glowing eyes are gold instead of blue.  With the new power, he destroys the rest of the Demons and closes the entrance to The Fade, and the form soon disappears.  Because of the massive energy it took to use the form, as well as the blood loss from also using Blood Magic, Spencer blacks out and requires medical attention, most likely getting the proper healing from one of the group.  He remains unconscious for some time, segueing into a later scrapbook.

Helpful things:
The Fade: http://dragonage.wikia.com/wiki/Fade
The ques the plot is structured after: http://dragonage.wikia.com/wiki/Night_Terrors
Lyrium: http://dragonage.wikia.com/wiki/Lyrium
Demons: http://dragonage.wikia.com/wiki/Demons
The Veil http://dragonage.wikia.com/wiki/The_Veil
Spirits: http://dragonage.wikia.com/wiki/Spirits
Battlemage: http://dragonage.wikia.com/wiki/Battlemage
Arcane Warrior: http://dragonage.wikia.com/wiki/Arcane_Warrior