I'm the Villainess, So I'm Taming the Final Boss, Vol. 5 (light novel) — Sarasa Nagase

Everything that could possibly go wrong does when a man claiming to be the first demon king and Claude's father appears without warning and promptly demands that he divorce Aileen. What's worse is that Aileen's alleged father-in-law, Luciel, is the one and only hero/final boss of the fourth installment of her favorite otome game series! The strange thing is that, even though the fourth game takes place several hundred years before the others, oddly similar events have begun occurring, so no one knows what could happen next!