Disclaimer: I wrote these words, I stole these concepts.
It's said that Azkaban drives a man crazy. It's the dementors that do it. Swooping and slipping down hallways and through bars, they creep into your cell and into your mind before you can even begin to erect defenses against them.
Nothing keeps the chill away—not occlumancy, not wandless cheering charms or conjured flames. Nothing. Relentlessly they slip into the mind and destroy the most cherished memories, the only brightest spots in a tarnished past.
Piece by piece the psyche fractures and you lose yourself, the parts of you that make you…well, you. After a few weeks, possibly months if you're very strong, all that remains is a shell inhabited by the Ghost of Darkness.
Every misdeed, every betrayal, every moment pushed to the back of the mind that you promised you would never think about again—these are the moments that one lives in. These are the only moments left that matter. This is who you are.
Sirius Black sat morosely in his cell, pondering the nature of the darkness that was growing inside of him. The simplicity of his canine emotions was enough to provide a buffer from the worst of the onslaught, but he could not always be a dog. In the interim, it was his parents to whom he owed the debt for his continued sanity.
Ironically, the very same misery and agony that had caused him to run away from home as a boy was responsible for saving him now. As a passing stranger gawked at the convicted psychopath, Sirius shut his eyes and watched the never ending stream of vitriol spill across his mind's eye.
He imagined that it was something like the muggle expression, "watching your life pass before your eyes". He believed that they claimed this happened when one was near death. Was he near death? He often hoped so.
He needn't worry about losing himself to the dementors. For every happy memory he had ever had, he had a corresponding moment of grief or agony to balance it out. Using this principle, he remembered everything. He remembered too much.
Authors Note: If I get enough response I will continue this piece. It will now be around 6 chapters if I follow the timeline I'm working from…but sometimes things get away from me. It will be followed by another piece about Sirius's reaction to Remus in the Shack.
PS: I am searching for a beta, if anyone is interested.