BASICALLY he makes a black magic sword-thing (or several) appear that he can control at will and that at full power would be capable of killing gods. As in that is literally what they were designed to do. He isn't at that level of power at his current his canon point, but he can take on weaker supernatural monsters single-handedly, could probably kill a human outright easily, and can effectively "erase" a spell by using them to destroy the magic involved in it. Canon is unfortunately kind of vague with the details but the gist of it is that they're a god's weapon intended to be used to slay other gods and supernatural-type things and can be used in the physical world to a degree as kind of an afterthought.