It's a blade from the Elder Scrolls series that was created specifically to harvest souls, and which would corrupt its wielder through its use. Very powerful, and very evil. More info through here: http://www.uesp.net/wiki/Lore:Umbra
Why am I bringing this up? Because I think that making a Champion who is based around a similar kind of blade would be an interesting concept.
Here's some rough lore I've made for a new Champion Concept I'm developing: (yes, this schtick again; think of it as an example)
Champion Name: Grolla Seyfarth, the Demon Blade
Former heiress of the noble Seyfarth family in Noxus, Grolla has become a ruthless murderer following the downfall of her family. Wielding the cursed sword Grollschwert, she has become a threat to all around her, possessed by the very blade she wields. Every foe slain by her blade feeds an unnatural hunger, not for blood, but for the very souls of mortal kind. And her greatest desire is the downfall of the current ruler of Noxus: Jericho Swain.
When Swain’s regime took control of Noxus, many noble families loyal to the former ruler Boram Darkwill were ousted, or otherwise slaughtered. The Seyfarths, a proud noble family of Noxian warriors, were one such family to face the slaughter, with Grolla being the only survivor. Desperate for revenge, Grolla journeyed to the Seyfarth family’s tomb where an ancient greatsword, the Grollschwert, was rumored to rest, sealed away by the spirits of her ancestors. Ignoring the warnings from her departed ancestors of the sword’s evil power, Grolla found the Grollschwert and removed it from its sealed bindings. But upon grasping the black blade, she learned the cost of her actions as her very soul was stolen by the sword’s curse.
Emerging from the tomb, Grolla was confronted by a task force led by Darius, the Hand of Noxus, who sought her head. Grolla was merciless, cutting through the Noxian elites like a scythe to chaff, leaving only eviscerated remains of her foes as her blade harvested their very souls to feed its hunger. She allowed Darius to live, albeit wounded, to inform Swain that she would eventually come for him, and that his soul would belong to the Grollschwert in time.
This blade has claimed countless souls, including my own. I will make do without one. – Grolla
Interesting dimension to explore, or too cliche?