Guys can u tell .e which manhwa is this : the fl gets back to past and then she tries to h...

Guys can u tell .e which manhwa is this : the fl gets back to past and then she tries to have a night with the ml ( duke ) so that the son ( the hero who would kill the monster in the future ) would be born . In her first life the monster takes over the world or something and there was no hero to stop it cause the son of ml was suppose to kill the demon but our ml was never married. Well can u tell me !! The plot was along these lines!! I guess