I don't think it will be Marcel and Davina I think the Big Bad of the first season will be who ever it was that is responsible for Cami's twin dying I mean if it was Marcel they would have just said so I think it's more likely that Marcel new about the massacre and knew Davina could draw power. Though I wouldn't be surprised if Cami's brother was some how still alive I mean a church massacre seems like it's very stanic and the word demon gets throne around now like a lot on both shows. Also Sean might be the show's version of Christian (Meredith's twin the books).