The Golden Age.
Kozmotis fought against creatures of darkness many times to defend the Golden Age, and eventually rounded up these dark demons into a cage. At the time, they were known as fearlings, but you might know them better by the name of 'heartless'.
Although General Kozmotis Pitchiner stood strongly against the darkness for many years, it was the love of his daughter that brought about his downfall. The heartless in the cage he had been guarding had a most unique ability to morph, and when it one day noticed a picture of General Pitchiner's daughter that he's kept in a locket, the heartless took the opportunity to trick the General into believing his daughter had been trapped in the cage and needed to be set free.
He wouldn't have fallen for it, but it had been so many years since he'd seen little Emily, and the time had made him desperate to see her again, wishing the creature in the cage to be her, and not some monster in disguise.
As soon as the cage was unlocked, Kozmotis was attacked by the powerful heartless, and soon lost his heart to it.
The shell of a man that was left behind, the nobody, lived a half-life for many years, never aging, never feeling emotion, never really being human. A thousand years must have passed like that. The heartless had left an impression on him, and since the heartless were all he could remember, he took them as allies and helped them to destroy the Golden Age, no longer retaining the memories or desires that had kept him protecting it for so long.
Over time, he came to know himself and others came to know him by the name of Pitch Black, and he was feared by all.
That was, until a young man and his two companions appeared seemingly out of nowhere, defeating the heartless that plagued the world and paving the way for the Guardians to come in with all of their hope and wonder and...dreams. Pitch thought that he should resent the group of friends who had taken down his heartless, but instead, he was simply jealous.
Pitch had seen much of the three, and it became obvious fairly quickly that they were childhood friends. They thought of one another as family. Pitch could feel a lonely ache that pushed him to want more. More of what, you might ask? He couldn't respond to that. He didn't know yet. For the next many hundreds of years, he plotted a comeback and prepared himself a heartless army in the hopes that regaining what he had before Sora, Riku, and Kairi had appeared and ruined everything would somehow make that feeling of yearning go away.
It didn't work. Jack Frost appeared. Pitch had initially thought that he and Jack were the same. That Jack felt that emptiness, that loneliness that Pitch had been feeling since he could remember. He wasn't completely wrong, but their differences tore them apart. Pitch believed that the only way to fill his empty heart would be through power, and Jack knew that he just wanted companionship.
Pitch wanted companionship too, but he didn't know that. He didn't understand. He thought he didn't have feelings.
So why did the thought of the close bond between those three friends bother him so much?
((Author's Note: Sooo, yeah. I was trying to write the next chapter of a different fic when this idea of Pitch being a nobody popped into my head and wouldn't leave until it was written.))