Rings of Power/Three Pines Crossover AU...
Adar and my OC (his wilder, less stable, blonder twin brother, Bainnir) are kidnapped by a serial killer (Morgoth), and are raised underground, in old sewers and abandoned train tunnels. Over the years, more kids are taken, until Morgoth has a small army of children with homicidal tendencies.
Eventually, Morgoth disappears, believed to have been caught by the police and the oldest "brother" (Sauron) takes over. At first, things are mostly the same, but then Sauron becomes drunk with his own authority, and takes the family in a different, more dangerous direction. He quickly puts down any dissent, and treats his siblings like slaves, even punishing and branding them.
Fed up and fearing for the lives of his family, especially his brother, whose illness and disability make him look weak to Sauron, Adar kills him (so he thinks) and takes his family out of the darkness, to try and build a different life. Enter Inspector Armand Gamache and his team, who quickly get involved when it becomes impossible for Adar's family of homicidal, unsocialized people to blend in. Gamache's team investigates, trying to figure out who these people are, while all the while, Sauron plots his revenge.