The plan didn't work...

She trained her entire life to be the best warrior she could be so that her best friend and sister, the first Princess, never had to worry about being safe. The first Princess is in secretly in love with her boyfriend, and comes up with a plan to trap her so she can be used as a sacrifice to a mostly forgotten love god. Except the mages messed up their translations of the old spell, and summoned the Demon God, who upon being faced with a beautiful, fearless, enraged princess fell in love. When he asked her what she desired most in the world, she told him "Whether it is in this world, or the next, I will make sure my sister pays for this betrayal. They will all pay." Because she knows that her parents had to know something about it, otherwise the best royal mages wouldn't be there. He agrees to grant her the power to get her revenge, if she would be his Queen. She says yes.