Well Here goes... hope readers like it.
Part One:
The League Journals do not address this, mostly because this has not been recorded. Karthus as a child lived in the City-State of Noxus. Of course, due to his fascination with death, he did not have any friends except for one named Amoline, who was a orphan. Through their teenage years, the pair were always together, Karthus a singer and Amoline a bartender, which allowed them to get by. While he was with her, his fascination towards death would ebb, and they were happy together. When Amoline was 16 and Karthus 14, Amoline was created the Grey order along with Gregory Hastur, and Karthus was invited to join. Karthus had loved Amoline, and Amoline herself was not fully aware of her love towards Karthus, but had left Karthus and decided to marry Gregory. Karthus, in a fit of rage, tried to kill Gregory and was beaten to a pulp. This ended the friendship between the pair, and soon after Karthus found his obsession clogging his mind again, and made his way towards the Shadow Isles. This journey took much longer than he expected, and while on his way there he again encountered Amoline, who had silently longed for him. For a month, they stayed together, Karthus was not missed, and Amoline was gone under the pretense of recruiting a resistance to eventually take down The Noxian high command. She left him the next day after a heinous argument over Karthus venturing alone into the Shadow Isles, and never saw him again. She was not alone that day. Karthus, depressed, continued towards the isles, and soon fully became the lich we know today, and Amoline soon gave birth to a child, named Annie. Six years later, Gregory and Amoline were put down by the Black Rose, and its secret weapon, the former Command General of the Noxian military, reborn again. Amoline's last words were that Annie had to go find a man named Karthus, but the lich had already heard of the attack from The Chronokeeper, and had come to find destruction, only to find a crying girl and her teddy bear, and so Karthus revealed himself to her, for even a lich has a heart
Poor Gregory never found out that Annie wasn't his child
End of part one.