We've datamined a Patch 9.2 cutscene in which Sylvanas reflects on her past with Arthas.
SPOILERS!!! Possible Major Story Spoilers Ahead.The dialogue in this cinematic was previously datamined. At the time, it was unclear if Sylvanas was discussing breaking free of Arthas or the Jailer, but with more context, it is clear she is talking about Arthas and breaking free of his control in Warcraft III:
I don't remember the exact moment I wrested control. I do remember the rush of freedom. The Read more on wowhead.com