The MC transmigrated into a novel and became an extra from the novel. The novel is a murder thriller and the main character is a serial killer. He kills criminals with black magic making him a dark hero (think of him as Dexter or batman with dark magic).
She (MC) doesn’t have any death flags since she’s a background extra but she was a homeless girl of all things in such a dangerous novel. Somehow our MC saves the male lead and to thank her, he adopts her. “Become my daughter.” The richest, most powerful, and handsome duke commands her to be his child.