Okay, so I have a theory that everyone may be interested in. Now, obviously Tartarus has been around in the world for awhile. Obviously for them to become one of the three main dark guilds in the Balam Alliance they had to have started somewhere. But what if it started with just E.N.D? He or she is the guild master and the most powerful demon of Zeref's Book. So what if he or she went around collecting the Nine Demon Gates. Or something similar. Bearing that in mind, and the fact that no-one can deny how much Silver looks like he's related to Gray. What if he IS Gray's father? And the only reason he's one of the Nine Demon Gates is because what is currently happening to Minerva, happened to him? Gray's parents were killed during Deriora's rampage. But what if Silver (possibly Gray's father) survived, and because of that E.N.D decided to have Kyouka turn Silver into a demon and furthermore one of the Nine Demon Gates. If you think about it like that, it's very possible that Silver IS Gray's father, and that we can expect some huge, heartfelt moment between the two...or as tradition goes, one kick-ass fight between the Ice Mage and the Ice Devil Slayer. Just something to ponder, guys. But again, it's very possible.