"Not triggered in most of try and I don't know why" - Evelyn's large secret is one of those where after you've revealed the secret and after you end the remove inhibitions spell, there's a dialog option to talk to her about the secret you just revealed (you always want do that when given the choice). If you don't talk to her about it separately from revealing the secret, she doesn't learn that you know she's in the Order of Thrones, and thus you don't get invited to help her arrest Thaddeus during the endgame.
"what does it mean" - If you haven't revealed all of Evelyn's secrets, then when you go with her to arrest Thaddeus, bad things happen. When T mind controls her and tries to make her cut her own throat, the MC uses invade mind to stop him. But if the MC does not know all of her secrets, this ends badly for Evelyn (the more secrets you've skipped, the worse the outcome for her). The worst outcome, however, is when the MC hasn't fully trained his magic skills. In that case, Evelyn dies, and the MC kills Thaddeus in revenge, which gets him thrown in jail, game over, but you get the achievement.