Do you know what happens when a child is born in Azkaban?
That child is born without any happy memories. It is unable to create any new ones so long as it is around the Dementors who reside within and guard Azkaban...
Joy is the most primordial of feelings in humans, and one cannot have any other without it. Without happy memories, a child cannot even form bad ones.
At some point in childhood, this inability to create either happy or bad memories becomes a permanent state.
It is at that moment, when the child becomes known as a Souless, an odd thing of inhumanity. A neutral creature, who knows no love or joy, no hate or anger, just... nothing.
The Dementors, wraithlike dark creatures will then recognize the Souless to be one of their own. They see the Souless as creatures of darkness like themselves, whom they cannot feed upon, but need not worry about. The Dementors, like the Souless, hold not joy of their own, at least not like humans can, but if one was lucky, or rather unlucky enough to see a Dementor and a Souless interact, you would call them both happy.
There is only one Souless known to humanity- born nineteen years before Azkaban was discovered in the 1500's, a little boy named Tartarus.
He was born on the island upon which Azkaban was built, to an unnamed mother who died in childbirth. His father, Ekrizdis, was a madman, who practiced the worst kinds of Dark Magic. He lured muggles to his island, and the Dementors who infested it fed upon their souls, allowing Ekrizdis access to the magic born of murder and death.
Poor Tartarus never knew joy or any emotion at all. And after a point, the Dementors began to recognize him as one of their own. When Ekrizdis died, the Dementors all but adopted Tartarus as a Dementor himself, and they cared for the Souless, until he died from unknown causes two years later, before Azkaban became the prison it is today. They say Tartarus's soul went on to become a real Dementor, but nobody cared to ask the foul creatures if this was correct.
As time went on, people forgot about the Souless, and what kind of truly inhuman creatures they were. They became legends- the emotionless creatures akin to the Dementors. Tales to scare little children into sleeping at night. If you don't go to bed, I'll hand you over to Azkaban to become a Souless. As if the threat of Azkaban wasn't enough. And still, people forgot they existed, that those horrifying creatures were real... And even later, people began to forget about the Souless all together. Tartarus was lost to time, the raging waves, and the ghastly creatures that surrounded Azkaban.
But, one particular night, a Pure-blooded couple joined the ranks of the prisoners of Azkaban. One was innocent, one was not. The innocent was named Sirius Orion Black, who was falsely accused of murder and association with Lord Voldemort. The other was his girlfriend, named Janet Sally Fawley. She was actually a spy for Lord Voldemort, and unknown to everyone save Janet and the Dark Lord himself, she was the last Pure-blooded descendant of Salazar Slytherin.
She wasn't caught for that though, no, she was caught because she had been with Sirius when he went to kill the real traitor who sold out the Potters to Voldemort. She wanted to make a clean job of it all. She was unaware that she was pregnant. A fact she soon learned, while in the confines of Azkaban's walls.
Janet worried for her child, whom she knew was the next in a new line of Pure-blooded descendants of Slytherin. So, she did her best to keep her sanity, if only for her child. Each kick provided her with new happy memories for the Dementors to feed off of. She was the only one still producing them, and they flocked around her in their regular rounds around the prison.
But, eventually, her baby was born. And, being away from any proper medical facilities, she died giving birth. The Dementors, however, recognized the emotionless child within her cell. It wasn't a Souless, not yet, but they still felt the budding kinship between them and the infant girl.
So, they took care of her, and raised her, in her dead mother's stead, hiding the infant from the human guards who came near.
It was ten years before anyone noticed the child- but she had already become a Souless... It was too late for the poor creature to ever have a chance of feeling again.