Fat Princess, Demon Queen

An office worker collapsed at her desk from overwork and woke in the demon realm, in the body of its eldest princess. The body was a good deal heavier than the one she had left, and a system had attached itself to her, one that paid out rewards for what she spent and what she finished. She meant to hold the position quietly, but the fight over the succession pulled her in. Her younger sister wanted the throne. The princess worked through the plots against her one at a time, rose as she went, and gathered allies and consorts around her. At her coming-of-age ceremony her sister's claim was broken. Her sister's answer was to foul the root the world runs on, and the damage from that went well past the court.