The original God Killer Blade was an ancient xiphos sword wielded by the demi-God Avem and used to seemingly slay Enyalius and start the Twilight of the Gods. When the Five came to power and brought stability to the world the God Killer Blade shattered, it's remains scattered and forged into lesser weapons.
Today the five fragments have been brought together and the God Killer Blade reforged by Gof Cadarn - this time in the form of a rapier.