Our protagonist has been transported to a world where humans and demons are at war.
In order to end this conflict and bring peace, she was offered a single special ability of her choice when she arrived.

If only she could get the Demon King and the hero who had been sent to defeat him together in one place to become united, nobody else would be hurt... that would wrap things up nicely, wouldn't it?
And so, she was granted the power to ignite passion in people - a "Force Estrus" spell.

During the final battle, she finds herself in the Demon King's castle with the hero and the Demon King and casts her spell.
All she has to do is watch over them and make sure it all plays out... but she is discovered from her hidden viewing spot in the next room.

Not only was she discovered, but the passions awakened in those two became focused on her instead...