It's been a while I wrote non-numerical things and English is hard.
Prologue
[Between Year X791 to X792]
The dead leaves crunched underneath her foot. It was late summer, the majority of the trees were still lush with greenery and filled with life. She gingerly followed a little path of dirt, where the line between life and death mingled and blurred.
"You came." He said plainly, eyes remained focus on the pair of stone façade double doors in front of him. The façade had seen better days but now disfigured by Father Time. The gold decorations that once framed the door were scraped bare by greedy by-passers; ornaments that once decorated its main panel scored chips and scars. These doors seemed rather ordinary and unimpressive for a tool that held the great magic for time travelling.
"What's on your mind?" Anna Heartfilia took a seat adjacent to Zeref, although she maintained a comfortable distance from him in view of how everything surrounding him is now lifeless.
Zeref kept his glance on the doors, "I am curious… whether my next course of action would bring me deep regret. But, I don't have the power to see the future."
Anna wondered whether he was referring to his steadily strengthening of the Alvarez Empire, or regarding the violent and rapidly dominating terror known as Acnologia, but she didn't dare to ask. She replied softly with a confident smile, "I have fate in this generation."
"I don't." Zeref finally glance at Anna, his face stoic.
"Since the game, I regret every day that I didn't close these doors once the 400 years plan was accomplished. I am worried what else could pass through the doors." She had witnessed the horror from afar during the Grand Magic Games, she understood how these doors could be misused if fell into the wrong hand.
For the first time since Anna arrived, Zeref let out a hum and the corner of his lips lifted. The smile came as quickly as it disappeared. "Is that why you visit these doors so frequently? The door is rather small for another dragon to pass through, if that's what you are worried about."
Unimpressed by Zeref's indifference and mockery, Anna held onto her doubts. However, there was nothing she could do now without any of her celestial keys. "Would you ever want to close them?"
"Maybe, maybe not." Zeref dusted himself off from the ground, "shall we have a bet?"
Zeref never walked pass the doors to rewind time in year X792, despite losing his bet against Anna. So the second door stayed open and contented with magical energy, hidden and undisturbed in a forest within Fiore.
Until one day, someone with a malevolence intend passed through the gate, searching for Zeref.