Crossovers That Should Not Be 27 - three sets of three rules of three... stinkers

[ECSNorway]

...  Farslayer, that's one of Zelany(?)'s 12 Swords, right?

IIRC, you choose a target, throw it and it flys off and kills them.  It does +not_ come back , so now somebody around your target has  the sword. 

That would lead to an awful large death toll fairly quickly in Westeros...

That's the one, yes. And that's exactly what tended to happen every time it was used in its own canon.