This one of my biggest (and somewhat crazy) Theories yet, please no bashing, enjoy!
What We Know
Ok, so what we know is demons such as Deliora are above average Mages such as Ur, possible Gray, Natsu, Lyon and others, but he is out of the picture thanks to Ur's last resort spell "Ice Shell".
The Nemesis Hades summoned are ludicrously strong, we didn't see any one beat, though they were destroyed when Hades was weakened.
Then there is Doriate, who he was able to weak three of the best FT Mages and almost took down two. Doriate was almost invulnerable to Gray's ice magic even as a kid, be he was able to defeat Doriate with the ice magic that he channeled through himself from the ice that froze the village, and also the Giant village was frozen by the Demon and Ice Demon Slayer Silver.
There were several theories saying that Gray could be Silvers son thus making him a half-demon.
And also that Silver might betray Tartarus.
What if these Demons they are all but invincible that almost no one can beat them (except Zeref and possibly Acnologia), they are literally immune to human used magic. And what if Silver is conspiring against Tartarus by using his own son Gray by embedding into him Ice Demon Slaying Magic so that he may sacrifice himself so that he may give it to his friends in order to beat Tartarus! For Gray is the Chosen One to save humanity?