Ah! I had to look it up because I couldn't remember, but according to Grisha Wiki (which I'll believe, I guess) it's actually a plot line from Siege and Storm, and that's what turns her against The Darkling.
I can't say for sure, since it's been literally years since I've read the original series.
I wouldn't be surprised if they take some elements from KoS though, since it's mostly about her and Nikolai's POVs.
