The Grudge Iron is a supernatural weapon that quietly accumulates energy from the despair of those who were unjustly killed by other swordsmen. When their cries bubble over in to a rage, the Grudge Iron manifests from the shadows, rotting wood and tarnishing iron in its wake. It screams out in vengeance at the murderer, proclaiming their crimes and demanding that they be punished for them. The sword does not kill its target, it simply pursues them relentlessly until they die of fear, exhaustion, confusion or maybe an accident as they try to escape.
Mentioned in Hellswing the Witch Hunter's Cryptid Casefiles.