The Idea Factory - The Casting Caste

There is a two caste system. The ruling caste can cast spells, the other supposedly can't and is practically slaves. During a party, several members of the ruling caste die, and one of the slave caste stepped into their place. She could barely cast at all, but rose to be the empress. MC is her daughter, several years after. She has no idea of this and thinks she's just a member of the ruling caste. Her father was and she can cast a little. But magic is waning and the ruling caste is trying to hide this. MC is good at tricks and uses them to pretend that she's better at casting than she is. The empress uses another caster who is still strong with magic. But then, with the empress's failing mental health, it is all discovered. The MC must deal with her crumbling world as rebellion rises up among the slave caste.