Forged by the smith god Vaul for Khaine, the Elven lord of murder and violence, the Old World knows of no finer blade.
The user becomes powerful beyond measure, with it's greatest wielder Aenarion once using it to kill four Greater Daemons solo, and many wielders in the Total War: Warhammer timelines have been able to use it to easily slay entire armies on their lonesome.
But there is a price: the more one uses it, the more the blade corrupts the wielder, making them callous, cruel, murderous, and ever more like Khaine himself.
While called a sword, the weapon actually reshapes itself to the form the user most desires at any given time, as it is willing to do anything to tempt an unfortunate soul into wielding it. It always shows a little of its true nature however, as whatever form it takes, it seems to be dripping blood.