END & Natsu: Power & Memory Theory
I have this theory. Like a whole lot of the ones spinning around the fandom nowadays, it's about Natsu and END. I'm not sure how to start this off, so I'll try the overview. (the picture means nothing, I don't have any suspicions or speculations about Dragon Force)
My theory suggests that Natsu and END are not two different people in the same body, or that END is the "demon laying dormant within the great Natsu Dragneel," or whatever. I think that the two are one in the same. The only difference is the knowledge and power they hold. Let me explain. It is confirmed in chapter something or other, when Zeref talks to Natsu in Tartaros'sheadquarters, that Igneel tried to defeat END, but failed. What was it? Ah! Chapter 373. I hope your satisfied. Anyway... The circumstances of their encounter is unknown. I think that END and Igneel engaged in battle of the traditional sense, but the dragon was overpowered and failed to defeat the demon. It is not mentioned what happened after, by my theory involves either Igneel or Zeref himself weakening END and sealing his memory and power away inside his book. Remember, END stands for Etherious Natsu Dragneel. That evidence implies that END is Natsu. Following the containment of END's memory and power, reducing him to an infant-like state, Zeref may have entrusted Igneel to care for his brother.
Tartaros & END
Part dos! We know that Mard Geer somehow got his (filthy) little hands on END's book, but we don't know how. My theory doesn't delve into that, but it does provide insight on END's relationship with Tartaros. I can't determine if END and Mard Geer met personally, but if they did, here's what I think. Considering the fact that Mard Geer didn't recognize Natsu or his name in battle, it can be assumed that he was never addressed by his full name, Etherious Natsu Dragneel. This also means that he took on a different appearance and possibly persona.
Regarding Face, I don't think that anyone in Tartaros could have "awakened" END, regardless of whether the plan worked or not, for this should be something that only Zeref himself can do. Why Tartaros wanted to activate Face could have another reason. Perhaps they thought Zeref would be pleased if they wiped out all wizards on the continent, maybe even all of Earthland. Let's not forget that, as of the previous era's end in X784, Zeref's goal is to destroy humanity. Maybe the Etherious knew this and wanted to fulfill their masters wish, as well as terminated all magic on the continent, allegedly supplying them with enough curse power summon END.