In light novels, characters like this often drive the story forward in interesting ways. The 'last boss queen' might start off as an enigma, with her true motives and powers gradually being revealed as the story progresses. Maybe she's not truly evil but is just trying to break free from the constraints that the world has placed on her. This could make for a very engaging read as readers get to discover more about her along with the other characters in the story.
The 'most heretical' part could suggest that the queen's actions, beliefs or her very existence goes against the norms or dogmas of the world in the story. It could be that she challenges the established religious or social order. For example, she might have a different view on magic usage compared to the mainstream, which is considered heretical. This could lead to all sorts of conflicts, both internal within the story's society and external, like battles against those who oppose her.