In my previous blogs, I stated my theory about Acnologia being the human-Etherious hybrid bastard son of Mard Geer Tartaros. If you look at one of my previous blogs, "H.E.A.T.", you'll notice similarities between Mard Geer's Etherious form and Acnologia's Dragon form. The lines on their bodies, muscle tone, arrowhead chin, feathers, ect. Am I the only one who noticed? Yes, Dragonification is defiantly the reason for his "transformation", but his appearance could be something he inherited from his father. Also, the lines on his body are also visible in his human form. There's no way he got those after gaining his transformation ability. It could be a birth defect for being only half Etherious. And being half Etherious could be what gave him the ability to change between both forms like an Etherious. "Ryuuousai" is the Banquet of Dragons, Humans and Demons. It'd make sense and also be symbolic if Acnologia was all three.
Now, the possible back story I thought of in "H.E.A.T." wasn't great but now there's a possible candidate for his mother. Anna.
Here's my new backstory. Assuming that they had guilds is the X300's, Anna took up a job request from her guild to help rid a village of a demon terrorizing it, which turned out to be Mard Geer. The two of them faced off with Anna's Celestrial Spirit Magic proven useless against Mard Geer's Thorn Curse. Eventually, Anna was entangled in Small Thorns and Mard Geer proceeds to rape her to know what humans feel like on the inside before tearing her inside out. In the struggle, Mard Geer breaks one of her Spirit Keys, summoning the Celestrial Spirit King, which would also explain how they met. The kings clash while Anna barely escapes with her life. She returned to her guild a week later to discover she's pregnant.
After the baby was born, Acnologia, or "Logi" for short, grew at an incredible rate, reaching his 20's by the age of 12. When he reached his 30's, he stopped ageing. In addition, he was gifted with immortal strength. These factors and the markings on his body lead Anna's clan, aside from her fellow guild mates who accepted him, to believe that Logi was a demon, and so, he was an outcast. When the Dragon Civil War began, Logi decided to learn Dragon SLayer Magic to earn the love and respect of his clan. He and several clan members enlisted. They returned on occasion as war heroes to celebrate their "victories" by bathing in the blood of their victims. They become corrupted by their power and Anna begins to worry about her son.
Eventually, Logi learned of what Mard Geer did to his mother years ago and, against his mother's pleads, went AWOL on a quest to find the Underworld King and kill him. Using his Dragon scent, he was able to track Mard Geer down, and eventually found and challenged him. The two fought with Logi easily dodging Mard Geer's Thorns and landing heavy "Iron Fists" in his face. Mad Geer asked what Logi wanted and the Dragon Slayer answered payback for what Mard did to his mother. Mard told Logi exactly what he did to her to anger him. Mard then asked Logi about the odds of him being his father. This makes the Dragon Slayer laugh until Mard entered his Etherious Form and the former notices the markings on Mard are the same as his own. Mard Geer takes advantage of a distracted Logi and briefly gains the upper hand until Logi enters Dragon Force and beats the shit out of Mard Geer. Mard lays on the ground in his human form and Logi, now knowing the truth decides not to kill him. Instead he leave him for dead and begins walking home, but his fangs, scales, and claws haven't disappeared.
While Logi was gone, Anna met the Black Wizard himself, Zeref, in the woods. After a long conversation, the two exchange each other's secrets, Anna explaining her Dragon Slaying son and his father and Zeref stating his identity, to Anna's dismay, and he was the one who created Mard Geer and the other Etherious. He then introduces her to his brother.
The next time Logi and his fellow clan Dragon Slayers planned to rebel against their fellow Dragons. The Slayers lead by Logi killed countless Dragons, including the ones who taught them. Logi kills his Dragon, and his fellow Dragon Slayers out of rage after a fight with them.
Logi returns to his village with horror in everyones eye. Anna tells his son about her encounter with Zeref and "what he told her". Logi, misjudging Anna, thinks she's a Dark Mage. Logi, now feeling betrayed by his friends, shunned by his clan and knows that everything he knew was a lie, rages mad and becomes a Dragon and laid waste to the villag, declaring himself "The Dragon King", reflecting his father the Underworld King.
After reaching the mountains where he spoke to Zeref in the chapter "Memories", Acnologia discovered his ability to change between human and Dragon like an Etherious. Once again an outsider, Acnologia officially cuts himself off from humanity.
Days later, stories of the Black Dragon stretch across Fiore. Anna, seeing no hope left for her son, reluctantly agrees to Zeref and Igneel's plan.
I dunno. Something like this.