User blog comment:B214/Light Novel Volume 16 spoilers/@comment-1392308-20131019074749/@comment-1359322-20131019085156

Yeah 666 would probably be the last boss. I was kinda expecting a scenario like this:


 * Team DxD gets the upperhand in their battle against Rizevim.
 * Cornered Rizevim manages to unseal 666.
 * 666 revives and when Rizevim orders 666 to annihilate Team DxD, 666 kills Rizevim instead, and then turns his attention to Team DxD.