sure you can join but you can not god mode and you start off as rank 1 like everyone else, also you take orders from any of the generals but if you want the dark god to give you tasks i dont mind but he doesnt show himself, he may send a messanger or familiar
also if your a dark priest you will have sort of demonic magics for healing demons or draining blood from enemies and healing with other people life force but you would not be too hardy or strong in melee, a priest is a flimsy magic wielding healing type character