I need everyone who didn’t read the books to know that Felassan (who’s name means slow arrow in elven)
Who was in several of solas’ memories and has notes around the lighthouse and crossroads
Was in the masked empire. He was a friend and mentor of Brialas and helped her with her efforts to use her relationship with Celene to improve the lives of elves in Orlais (and tried to let her down gently when it became apparent that was not enough).
He refuses to let Briala tell him the code for the eluvian network so that he cannot give it on to Solas. Refusing to tell him would not be enough to keep it from him. When he meets with Solas he tries to convince him that modern elves should be given a chance. Solas murders him, his ally of thousands of years, for this.
In inquisition, Cole says this:
This is what the Betrayal of Felassan represents. Solas murdered his trusted friend and ally because he though modern elves were people whose lives had value.