The only thing you need to know before you start is the basic Harry Potter story and that this takes place before the events of the RotG movie and during Harry's second year, and onward. That is all.
Harry had never feared the dark or the things that lurked inside it, the dark held no fear for him the way it did for other children. The dark and the shadows had always helped him, hiding him from his relatives and the children of the neighborhood when they wanted to hurt him. Harry heard stories from his cupboard sometimes, the walls of his cupboard where very thin and his Aunt Petunia would tell Dudley stories about Santa Claus and the Tooth Fairy and other legends, either not knowing or not caring that Harry could hear her too. He always believed in legends, where would they come from if there was no truth in them to begin with? The only presents he ever got were the annual gift from Santa he would find in his cupboard and hide before his aunt or uncle could see, he always found the most eggs on Easter even though he was forced to give them to Dudley or be punished, and he loved playing in the snow when it came each winter, not overly affected by the cold for reasons he didn't know. But there was one of his Aunt's stories that stood out the most, the only one she had told him directly to scare him even if hadn't worked, the story about the Boogeyman. Petunia told him horrible stories of how he hid in the shadows and under the beds of naughty children, giving them nightmares, and that he would come for Harry if he didn't behave. Harry's belief had only gotten stronger when he learned magic was real, and all the fantastical things it was capable of.
At twelve years kids were starting to grow out of the stories and legends they were told when they were little, no longer seeing the sheer magic in everything that Harry did, most of them having grown up with magic as a fact of life, and so he never told his friends about the trips he made into the forbidden forest. Harry snuck off to the forbidden forest every chance he could, finding all the shadows and darkness comforting and knowing that nothing in the forest would hurt, he had already befriended a couple of giant spiders during his exploration of the forest. This is how Harry would soon come to meet Pitch Black, the fearsome Boogeyman, and, shortly after, Jack Frost, the very spirit of winter.
So, that was the beginning of my first RotG crossover and I'm going based off the movie, not the books. Which I still can't find so I can read, dammit. It's short, I know, but they'll get longer, I promise.