User blog comment:Wrath022/Chapter 516: The Truth About Enchantment, Review/@comment-4507125-20161231163540/@comment-25111584-20170101202050

@Serena: There is no real world equivalent to the Dragon Seed. Suffice to say that it basically mutates its hosts into Dragons. The antibodies presumably suppress the mutagens. Apparently, not even Igneel knew what the Dragon Seed is or the entire range of its effects. More will be revealed in Acnologia's backstory, I guess... or maybe not. :P

Demon particles/factor are/is something which allow(s) humans to take on demonic forms. Whether they permanently or temporarily turn the hosts into Demons depends upon... something we have no idea about. Could be quantity, method of infusion or who knows what. How Mirajane got those particles would be an interesting story which will never be told because Mashima cba. .-. However, given that there is at least one glaring difference between Minerva's conversion as opposed to Natsu's (Tome of E.N.D.) not to mention the matter of simple mutation vs. resurrection, we shouldn't really equate Tartaros' method to create Demons with Zeref's method to create E.N.D.
 * TL;DR: Demon particles/factor infusion used in Tartaros arc is quite different from the method Zeref used to create E.N.D. Until more info is available, speculating that they are related is fine, but there is no evidence to support it and some evidence to the contrary.

Demon and Devil are the same thing in FT. And Gray uses Ice Devil Slayer Magic, overuse of which causes insanity/corruption of the soul/descent into evil/whatever you want to call it. Don't overthink it (the irony...), it just means that he'll go bad. Demonization/Demonification is either one of the uses or one of the side-effects. Difficult to tell which.

Tbh, I'd recommend not trying to make sense of the whole Devil Slayer plotline. It's basically shoddy patchwork at best. Anything happening in it is purely for convenience, no matter whether it contradicts past stuff or not. .-.