The Warlord's Innocent Husband

An apocalypse-powered individual transmigrates into Serena Sutton, the unfavored second daughter. Her family favored her manipulative older sister; her husband loved the sister and tormented Serena. Serena is no pushover, agreeing to swap fiancés for money. Her new husband is simple, with a near-demonic Maniac son whose only weakness is his simple father. The cunning Serena and the Maniac son go from mutual dislike to joining forces to protect someone.