There's no epiphany. No grand moment of enlightenment that signals a change. Lenore can't even pinpoint the moment when she made her decision. She just knows that it felt right in her heart, and that gave her the peace of no longer having to spend hours debating over if she was making the correct choice or about to make a mistake that could be bad not only for her but also for Alaric and the people who work on the estate.
But Alaric said beforehand that he would do as much as he could to maintain the secret of her mark. Of course, he can't guarantee that the information won't be leaked. Alaric has a lot of power, but all the power in the world isn't enough to control free will.
