User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-82.199.216.49-20150906113314/@comment-26865730-20150907233259

I don't remember there being any particular reason 666 would even want to fight GR? I'm guessing there would have to be some sense of self-preservation and GR should give anyone pause.

Unless it's just in 666's nature to fight for dominance or perhaps it's that way for dragon's in general?

Anyway, the 2 evil dragons at the end, Apophis and Azi Dahaka, had the Grail and AD stated his magic was better than Rizevim's, so maybe they can control 666.