Gain and Loss. I intended to write this as a "Seven" fanfic at first, but after a bunch of rewrites I decided to make it fully original instead. So basically, it's an isekai into played game with heavy kingdom-building vibes, set in a high-medieval European-style fantasy world (think feudal lords, duchies, arranged marriages, all that jazz). It is a grand strategy game that has the gameplay combined between building dynasty from Crusuader King 2, economy building from Victoria 2, and real time battle like Total War: Shogun 2 and TW: Medieval 2. The main theme is about kingdom (duchy) building, military and angst drama.
To summarize the plot, it is about the main character using his in-game knowledge to overcome challenges and him forging new relationships with his new family relationships as the NPCs are now real people and not text anymore. And later on, he had to deal with the consequences of cheating with another woman.
To a detailed plot, the MC is a mechanist who dies in a freak workplace accident and wakes up reincarnated into the exact world of his favorite grand strategy game... except he's dropped in about 200 years before the earliest start date (552 E.C. instead of 799 E.C.). His new family is a once-powerful noble house that's already dead and buried by the "official" game timeline, which is only remembered in scattered, negative lore snippets. That means he barely has any reliable future knowledge, no cheat-sheet events or big names to rely on. He's gotta build from almost nothing using what he remembers about regional resources, trade routes, geography, and general game mechanics (and it backfired some times).
The MC's name is Clem. He's born as the son of the current Duke. His mom is the third wife, who is a classic ambitious, scheming type straight out of those face-slapping novels, dead-set on seeing her boy take the house head title no matter what. His father is a typical cold, pragmatic medieval ruler who treats kids like political assets (though Clem slowly chips away at that worldview and gets him to care his siblings more). Having bad memories from his past life where siblings tore each other apart over inheritance, Clem's big goal from day one is to actually unite his siblings and half-siblings instead of letting the family implode.
In his teens he quietly uses his meta-knowledge to exploit untapped local resources and build up his own power base. The idea is to have enough independent wealth to support/protect his siblings once his father starts marrying them off for alliances. Along the way he meets and falls hard for the first heroine, Frostine. They bond through shared work and adventures, and it turns into a genuine, deep romance.
But at 16, he is forced to have arranged marriage to the second heroine, Gisela He has to break things off with Frostine to do his duty. The marriage starts purely strategic, but Clem's caring nature, constant protection, and using his foresight to dodge disasters for her and her people slowly wins Gisela over.
Years later tragedy hits: Frostine comes back into his life, old feelings flare up, and he slips into an affair. They get caught. The fallout is brutal as both Frostine and Clem end up dead in the mess, and it sparks a nasty, long-running feud between their kids and the kids from Clem's marriage to Gisela.