Primarchs were once the emperors children he made them before the Horus heresy. Needless to say half of them rebelled one of which was the emperors favorite son Horus.(as if you couldnt get that from the title of the heresy :3) The ones that lived through the final battle over terra became deamon princes under the chaos gods whom they joined after rebelling. As to whether there stronger im not sure, but since the chaos gods feared the emperor enough to actually work together to bring about his downfall I would say yes the primarchs are stronger then the greater deamons, But thats my opinion so dont just go with its fact lol