Aha! The turn to the dark side is full of thrashing and self delusion.
Quick world state catch up. It's close to canon-ish. Solas broke up with Eilan and left to pursue his goal of tearing down the Veil to restore the world to its rightful order. Eilan stayed on as Inquisitor. The Inquisition is now a direct military and ops vehicle of the Chantry lead by Leliana with Cassandra as her hand. The Inquisition's main target is Solas.
tl;dr: If you're wondering what radicalizes her.... nothing? She never agrees with Solas philosophically about the Veil or why it needs to come down. But she is so singularly focused on staying in a relationship with him, so self-absorbed in her love, that she makes a series of small, compromised decisions. Each decision feels, in that moment, absolutely human and excusable. But ultimately those decisions culminate in her corruption because they make her his accomplice. And once the deed is done, she engages in a series of mental gymnastics to live with herself and with him and her transformation from a do-gooder to a villain is complete.
The slightly longer explanation below: 👇
Eilan corruption arc. The story starts six months after Trespasser. Eilan and Solas were a couple and very much in love. She is convinced that if she could just find him and reason with him, he can be persuaded from his destructive path. She dreams about him, finds him that way (dream magic!)
He's actually thrilled by this turn of events. For him, it represents the possibility that she might change her mind and join him. He also wants this to end with his lover Eilan at his side. What a dream!
They both ignore the elephant in the room for a while, just enjoy being together and pretend they aren't at odds. But as he gets closer to his goal, she starts getting desperate and leaks some of his secrets to Dorian so that power players of Thedas can perhaps oppose Solas a little longer. But she never actually gives up any critical information that would help Solas' enemies stop Solas once and for all. That would be a step too far. Her heart isn't there.
Dorian, and others, doubt Eilan's loyalties. She had said she is Team Modern Thedas, but her actions speak otherwise. Dorian in particular feels betrayed by her. 😟
Meanwhile she's arguing with Solas all the time, their relationship is frayed very badly, she acts out, but he will not be dissuaded. He is convinced she will prefer life as an elf without the Veil and their lives will be better. He is also hoping that removing the Veil will make all elves immortal again. Fingers crossed, vhenan!!! You and me forever!!!
Finally, when Solas does take down the Veil, Eilan against all odds shows the fuck up and does have a way to stop his ritual. Bruh, it won't even kill or wound him. It is like, the perfect Solavellan gotcha: stop the ritual without hurting Solas? Wow, the dream.
SHE STILL BACKS OFF!!!!! She's too in love to disappoint and hurt him (emotionally) this way, and will absolutely choose him over the world (as her friends had accurately accused!!!) Sure, she gives herself a little mental band-aid about it, she reasons her decision not to stop Solas from *checks notes* destroying civilization as millions of people know it is about, uh, "having faith in him."
Nah, dawg, it's toxic, obsessive love.
That story is completed and published. Comes in at just under 30k. Read it here -> https://archiveofourown.org/works/38774526/chapters/96954213
I am working on the sequel which covers the first 3 years after the Veil falls. The draft is currently at 65k words.
In that fic, the consequences of the apocalypse are dire!! A little worse than Solas expected. You know. For consistency.
Eilan, at first, is not coping well and says ugly things to Solas. But in order to cope, she has to compartmentalize. Also, she's essentially in the most privileged position in all of Thedas at that point, and shielded from most horrors.
Meanwhile we see Solas start to have doubts about his choices re: the Veil (both putting it up and taking it down). His self-doubts start to eat at him and it is Eilan who builds up his confidence again. She's a playwright and writes literal propaganda to retell history where Solas is the singular greatest Byronic hero of all time. She tells him that what he did was right and good. Does she actually believe that? Doesn't matter! And not really? Look, they need to live.
Here's what she does start to believe: Some people just matter more to world history, you know? 😉
And her turn to villainy is complete without ever, even once, having a god damn point. She just wants her love.
I might write a part 3 where we see Eilan at her happiest. She and Solas are married, have kids, and the ugly past feels distant. In that story they learn that actions have consequences and the misdeeds of the past catch up to them both.