There Is No Need for the Tyrant Second Male Lead's Obsession

I've been possessed by a novel where the emperor, deprived of his beloved, becomes a war-crazed secondary male lead. And I'm just an extra princess in that nation swept away to ruin.To survive and protect her beloved father and people, she sought only one heir to inherit the mermaid bloodline's power... Yet the man she spent the night with was that very mad emperor.He feigned affection and obsession, then went one step further by treating her like a pregnant woman? Where on earth does this confidence that she's instantly pregnant come from? Please, just set me free!