Karthus is a terrible creature who was once a mortal so obsessed with death that he eagerly embraced the gift of undeath. Now in his unlife as a lich, Karthus is closer to his beloved grave than he ever imagined. He commands magic with oblivion as its source, and seeks to bring his grim truth to the rest of the world: only in death does life gain clarity and purpose.
Even as a boy, Karthus was different. There was a darkness within him that could not be denied. The young child clung to the shadows and shunned the company of others. He snuck into the funerals of strangers, and spoke to their tombstones in the cemetery. He sought out corpses of animals to preserve and memorialize them, creating a grim collection of morbid art. Karthus volunteered to tend to the sick and dying, though his intentions were not to help, but to closely witness the passing of those who were beyond help. Ultimately, he went so far as to elaborately stage his death, and he secretly observed his own funeral with intense fascination. When others discovered what Karthus had done, they were so shocked and disturbed that, in a way, he became truly dead to them. Karthus found himself fully rejected by the living.
His isolation only served to fuel his obsession. Karthus delved further into his exploration of death, and became fascinated by the legends of the Shadow Isles. There, it was said, the specters of the deceased continued on in unlife everlasting. Possessed by these tales, he knew he had to discover for himself if they were true. Journeying to those dreaded isles, Karthus soon found himself wandering through the mist, overcome by the surreal beauty of the place. He felt as if he had finally come home. He had always chased the elusive purity of the moment of death, when life passed and in a single instant achieved meaning. Karthus saw that undeath was like that moment, preserved in dread stillness forever. He had discovered his destiny, to cross over the veil and leave the living behind for eternity. Something awoke in the Shadow Isles that day, when Karthus did something no other creature had ever done: he willingly gave his life over to undeath. When he reentered the world, Karthus had become the embodiment of his own obsession. An undying lich with the keys to life and death, he seeks to bring his dark requiem to the world.
"There is no sweeter song than the last breath of the dying."
The vast quantities of physical magic that were consumed and destroyed during Valoran's numerous Rune Wars have scarred much of Valoran. Many locations far from civilization have been morphed into hazardous and dangerous locales. The Howling Marsh is one of these places, and the self-appointed overlord of this land is the lich known as Karthus. It is thought that Karthus was a mage who, in life, was foolish enough to enter the fetid waters seeking his fortune, and that he was forever transformed by whatever dark magic permeates the swamp. Karthus now rules over his swampy dominion with an iron (albeit skeletal) fist. Visitors are not welcome, and those who are not scared away by the lich's terrible power most likely end up as one of the undead themselves... skeletons and zombies in Karthus' growing army of eternal servants. On the darkest of nights, Karthus is said to sing the tales of those who have succumbed to the swamp, past and present.
Karthus' tenure in the League of Legends is one of the more unusual ones in the history of the League. First, it is highly unusual that a lich who was known for isolation and hatred of life would even be a willing part of an organization which promoted the preservation of life. Next, Karthus specifically eschews the accrual of power and influence gained through service to the League; he is essentially an unpaid volunteer. This behavior is completely different than what his life outside of the League would have onlookers believe. Karthus has never spoken about his intentions in being a champion of the League, and in fact becomes quite agitated when asked about it. In the interests of not aggravating a lich, most sensible people subsequently drop the issue.
"Come visit my home, and I shall sing a dirge of your life as it once was."