Disclaimer: The rights go to JKR. The title is taken from a chapter of the fifth book, of course.
Dudley never told a soul what really happened on that muggy summer night when he taunted Harry about the boy named Cedric. He didn't really know himself what went on; he only knew it had to do with Harry's magic, and someone evil, and two things called dementors. He asked Harry about it once years later, but he didn't really understand. Harry's face closed up and he explained a bit, but Dudley knew his cousin would never want to talk about it. Obviously, it was a bad memory for both of them.
It was funny, the way the mind worked. Dudley always remembered certain things about it that night as clearly as if it had just happened yesterday. He remembered he and Harry having a... conversation, for lack of a better word. It was the only kind of contact they'd had for years - insulting each other in every way possible. Dudley remembered Harry calling him Big D and Ickle Diddykins, two names he'd hated ever since. There'd been other names too, but he couldn't remember them. He also remembered how bright the stars had been, at least until they came.
Odd, though, that he couldn't remember what the name was of the boy he'd fought with that night, or which of his friends had been with him, or anything that had happened after he'd gotten home.
What he recalled most, though, was the event itself. The stars, the moon, even the streetlights had disappeared without warning. And then he'd been cold, incredibly cold, and terror filled his veins with ice, even colder than the fear. Then he'd begged Harry to stop, and when Harry refused, Dudley punched him harder than he'd punched anyone in his entire life.
It got worse. Things played in Dudley's mind, unspeakable things. The carnage he'd caused over the years. The broken nose he'd given Mark Evans when Dudley was eight and Mark was five. The guilt and remorse he'd learned to feel little by little as he grew all hit him at once, and the desire to die struck him more intensely than ever before. He felt something grab his wrists with a strong grip and he wanted to surrender, wanted to let what was coming just happen.
But a bright silver light appeared and whatever was holding onto him let go suddenly. Then people were talking and Dudley knew he was home. He vomited. More talking. He said one word - "him" - and then he knew nothing. He was awake on the couch, of course, and in the back of his mind he could almost hear his parents and cousin, but he was too occupied with trying to come back to life to pay attention to them. His head was aching fiercely, and there was a sharp pain in his chest, an unbearable pain.
The next day the pain and fear were gone as though they'd never existed, but Dudley knew better. He never forgot that night. His mother asked many times what had happened, but he never could get any of the words out - never really wanted to. He revisited it in his nightmares often enough without dwelling on it in the day, too.
Even Harry asked him, once, what he'd thought about when the dementors showed up. It was the day Dudley asked Harry to explain what happened. That was the only time either of them brought it up, as far as Dudley knew, but it was their common ground. In Dudley's opinion, that night was the first time Dudley ever was grateful to Harry and the last time he ever insulted him. He never mentioned Cedric again, either, though he was always curious about it.
Dudley never told a soul what happened the night the dementors came, but he never went a single day without thinking of it.