This is a blade pulled from material found outside the border of the universe, something made of nothing.
These weapons are extremely dangerous and Taboo, as it does not only kill, it erases someone's whole existence, no afterlife, no purgatory, nothing beyond, and then the blade disappears with the life of the person it was plunged into.
The erasing in some rare cases can be resisted, this causes strange things to happen, such as ones soul components being rewritten, splitting in two and making a clone, even godhood, but all of this is extremely rare.