Free: To Good Home! A Fight Club version of Albus Dumbledore. Dumbledore and Voldemort are the same person. Impossible? Yeah, well, magic. And time turners.
Dark Lord of Light
The moment his sister, Ariana died, Albus Dumbledore was broken. He couldn't handle the truth of what he had done. But, it wasn't until he defeated Grindelwald that his second personality, Tom Riddle, emerged from within his fractured mind.
At the beginning, Albus was the stronger of the two. The little orphan who tormented his classmates was easily cowed by the fully-grown adult who had been handling children for decades. But Tom was insidiously crafty, and slowly began gaining power over their shared mind. He avoided confronting the dominant personality, stealing control of their body for brief periods of time. Most often Albus never even knew he was missing hours.
It wasn't until Tom got a hold of a time-turner that things started to get out of hand.
It was quite a shock when Albus Dumbledore saw himself walking around as Tom Riddle, get sorted into Slytherin and build power and alliances. He knew what darkness and evil was there. He knew he should have himself admitted to St. Mungo's, knew he should stop the growing menace while he still was able, but to do so would condemn himself. He couldn't do that. Not yet.
Not ever. The birth of Lord Voldemort came the first time he took a life. When he made his first horcrux. It was also the moment Albus lost dominance. Lost the strength to stop his other personality through force of will. No longer was it just a matter of willingness, but will. Voldemort would not allow Dumbledore to reveal the truth.
But Albus Dumbledore fought back. Resisted. He formed the Order of the Phoenix to counter the dark acts that were being committed by the other personality. Dumbledore began using a second time turner to appear and confront Voldemort before he could finish his dark deeds. But it was only a stalemate. Neither could kill the other for they were one and the same. Voldemort was forced to flee, only to appear again later.
Only, Voldemort was growing stronger. The Light was slipping. Albus would find he had done unkind things, even cruel things. Tom was infecting him.
And then came the prophecy. The Dark Lord would be vanquished.
This would be his salvation. No wizard or witch yet living could match him. He lacked the will to do it himself. But if a boy could end him, if he could prepare that eventual outcome, that he could manage.
It was tragic how Lily Evans and James Potter died, but the resulting backlash from the bloodwards Lily Evans erected left only the one who wished no ill on their child, and that was Albus.
It was only a reprieve, though. Tom was not wholly gone. He couldn't be, not as long as there were horcruxes out there, and Albus didn't know where Tom had hidden them.
Harry Potter would have to be willing to sacrifice himself to allow Dumbledore to end, as the prophecy said. He had to make sure that it happened. For the greater good.