Pretty much nothing has been revealed about the Soul King's actual capabilities at this point, other than he maintains the balance of souls between the various realms. He has not been show to act in any offensive role, nor has he been shown to possess a Zanpakuto. In all his appearances thus far, he has not actually done anything despite being attacked, which suggests that he is either too focused on regulating the balance of souls or he is not capable of attack/defence. More might be revealed at a later stage, but at this point in time that is all that is known.