User blog comment:Xilinoc/Ch. 652 - THE THEATRE SUICIDE SCENE 6/@comment-1210805-20151119171334

So I've been thinking about the nature of Nanao's sword and I don't think it's specifically a "god killing sword". Rather, it's a sword that reflects (hence the mirrors), and as such, its power is more potent when fighting more powerful opponents, opponents that may appear godly in strength. Their power might make them immune to the attacks of others, but when their own power is used against them they're done for.