The wizarding world was shocked when after the war, instead of becoming a fabled Auror like his father, Harry Potter dove into work as an Unspeakable in the Department of Mysteries. He carried out his work like a soul possessed and only came home to sleep, much to the consternation of his new wife, Ginny Weasley. He dove into obscure magicks and magical lore most of the magical world had either forgotten, outlawed, or deemed unimportant. But any piece of information, any forgotten knowledge was important in Harry's quest.
He needed to know why since the war he felt as if he was fading from existence, why the world had gradually seemed to have lost colour and meaning, and most importantly why whenever he fell asleep (or more likely collapsed from exhaustion) he would dream what he could only deduce to be Voldemort's memories.
After spending another day scouring the Black library, he was no closer to finding his answer and apparated home, to the Potter manor. He was immediately assaulted by Ginny when he appeared, his arms full of his red-haired, fiery wife. She gave him a sultry smile and a pepper-up, telling him to go shower before joining her in their bedroom. After performing his husbandly duties he looked over at her and sighed.
'At least she's happy', Harry thought as he looked over at his wife's sleeping form, a satisfied smirk on her lips.
He knew he wouldn't be able to go to sleep tonight, especially with the pepper up still coursing through his veins. But besides that, he couldn't get comfortable and as much as he loved Ginny, being in her presence felt wrong. Sex with Ginny had started out easily enough, it was even enjoyable but recently he had just been playing the part and had reverted to muggle drugs to be able to perform.
Slowly removing himself from his wife's clutches he drew up his sleep pants and wondered down to Potter Manor's library. He hadn't explored it as much as he'd like, because whenever he tried to seek refuge within it, either Ron, Hermione or Ginny would find him and drag him out, insisting on his company. It was exhausting. But on the rare occasion when he was the sole waking person in his home, he sought refuge within the towering walls of his family's ancestral library.
He had been surprised by the range of topics and magical scale that existed within the library initially but came to learn that the Potter's, while being majority Light magic users, had a fairly neutral view on magic as a whole and his father had been the first to declare a definitive side in a magical war in centuries. It had surprised him, as he had often gotten the impression that the Potter's had been a solely light family, where everyone had been in Gryffindor since the line's beginnings. But as he had read some of his ancestor's journals, he had discovered that the Potter's had been in every house and it was only the last couple of generations which had been in Gryffindor.
Huffing in exhaustion as he reached the double doors of his family's library, he entered. Blinking tiredly at the expanse of books the room held, he noticed the stack of tomes that had been next to his preferred seating area had been put away. Huffing in annoyance he wandered around the library to find the tomes he had procured last time.
Surprisingly the Potter library had quite a vast collection of books on Soul Magick, and Harry had been about to start on his pile when he had been called away. Settling into what has been affectionately dubbed 'Harry's nook', he began with a nondescript brown book, where the title had long since faded. Thankfully, the inside of the book was well preserved, and the text looked as fresh as the day it was printed.
As he was reading the book, his previous languid posture began to stiffen and straighten as the words on the page captured his attention. His symptoms, his dreams… it was all being explained and what the answer was… Harry swallowed back his bile.
Soulmates. His soulmate was Voldemort.
He flew into a frenzy and began to read all mention of soulmates in the books he had collected, and then some as he continued to raid the library's Soul Magick section. All books confirmed his fears. He had killed his own soulmate. Upon this realisation Harry let out a truly agonised sound. A thought broke through the bleakness of Harry's thoughts, had Dumbledore known? Did his friends know?
A cold clarity came over him as a plan began to form, he would see if his friends had truly been in the dark. As for Dumbledore, he had a portrait to question.