Save the Demon King Volume 1

In a world of magic and war, a fierce Demon King formed a pact with the mortal kingdoms; every 25 years he will choose a human partner that he must try to love for 100 days. If he falls in love, the partner will have the power to kill him. Many
centuries later, the Demon King meets a gruesome fate at the hands of the girl he loves. His death sparks the beginning of the end of the world.

Aster, a farmer’s daughter turned mage, narrowly survived the apocalypse and now seeks an ancient magical entity to save what remains. But when she offers herself to this entity, she receives a vision from Maven, the deceased Demon King.
With what little remains of his magic, Maven sends Aster back in time to thwart the apocalypse. Like with all spells, there is a catch; she cannot reveal what she knows of the future.

When Aster wakes, Maven has already chosen the girl destined to kill him. Now Aster has 100 days to save the Demon King, only she never expected to fall in love with him in the process.