Modern AU. For the last twenty two years Uther Pendragon has been waging war on magic. Now his son Arthur has been framed for a magical crime and sent to the prison for magic users. Arthur is instantly targeted by the inmates, but mysterious top dog Merlin takes him under his wing. They form a bond, and Merlin decides to help Arthur clear his name.

such_heights prompted me with "some combination of Natasha, Pepper, and Maria, weapons training." This is what happened.