A traditional Nornic funeral sees the deceased laid out upon a funeral pyre with his or her weapons and belongings. A relative who wishes to avenge the deceased may reach into the fire and grasp one of the weapons. If the deceased was killed, and if the slayer yet lives, and if the relative has the strength to grasp the burning weapon and withdraw it from the flames, sometimes the fire remains on the blade, burning without any fuel.
Such blades are as honoured as they are feared. Almost impossible to sheathe save in specially enchanted scabbards, they are usually kept in display in a family's hall as a reminder that one of the clan lies unavenged. But they seldom wait long before they are used -- and a good thing too. For if the blade does not seek vengeance, it may turn against those who drew it from the fire with false promises.
The rules are simple: the burning blade must kill the slayer of the person from whose pyre it was taken. If more than one person was responsible, it must kill them all. The gods decide who is responsible, but the gods have a pretty simple view of responsibility -- they don't care about economic causes or earlier wounds; they want the person who dealt the death blow.
If the burning blade does kill the guilty party, the fire slowly fades. However, the blade retains a flickering glow in the metal, and from time to time it emits a low heat. Henceforth the blade will retain a righteous magic. Such blades are passed down through clans for generations, both as powerful magical weapons in their own right and as symbols of the clan's readiness to avenge slights against it.
If the burning blade does not kill the guilty party, whether because someone else did it or because the guilty party died through other means, the fire of vengeance has no righteous outlet. It begins to turn inward, consuming the bearer. Anyone who bears such a weapon after its target is dead begins to suffer from the mental effects of bearing it; the rage lacks an outlet, causing the wielder to lash out in unpredictable ways. Alternatively, the wielder may somehow lose the blade. Such weapons often find their way into the hands of clan enemies; no one is quite sure how this happens, but it happens often enough that it cannot be a coincidence.
Some such blades have been destroyed, although destroying magical weapons is always a tricky process. Others are buried deep in the earth, their fires banked but not extinguished.