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In Mithos, where white magic is fueled by purity and black magic by passion, True Love is known as the Intolerable Sin. It’s the most unstable and dangerous source of magic in the world. The punishment is simple: a quick death. Oooo, this sounds pretty fascinating.
Martia is a Love Child, the daughter of True Lovers. Raised within the walls of Siris Academy, she’s been taught to hate both True Love and herself. Now that she’s graduated, Martia is out in the real world, doing what the Academy trained her to do: assassinate those who’ve committed the Intolerable Sin. Looking good so far.
Then she meets her True Love, Narin. Martia refuses to fall under the lulling spell of the Intolerable Sin. She knows a mere caress between True Lovers could send out a flare capable of eating through skin and bone. But Narin's kind words and gentle smiles maybe not plural for "smiles" make Martia wish she could be more than a brutal executioner. When the Academy discovers her crime, she has a choice. Follow her training and kill Narin, or give into the black magic that betrays her past and risks her future.
This is a really, really good query. I honestly don't have anything to add. Your concept is interesting and seems original. The only thing I'd consider adding is possibly stating whether your MC begins to doubt whether her crime of love is actually a sin or not. She doesn't seem to be questioning her training, just whether or not to follow it.