Farslayer howls across the world
For thy heart! For thy heart! who hast wronged me,
Vengeance is his who casts the Blade,
Yet he will, in the end, no triumph see.
The magic behind Farslayer, the Sword of Vengeance, is certainly one of the most clever, deceptively simple powers of the Swords -- Saberhagen deserves a big pat on the back for this one. The delivery and follow-through add to the disturbing charm of this Sword.
Simply put, you swing Farslayer about (there is a traditional chant, alluded to in the song verse, but it does not seem to be required), and once you designate a target for your vengeful strike, it proceeds to fly forth from your hands and embed itself in the heart of that target (man, beast, demon, or god), no matter how far away he/she/it is, and no matter how hard they try and hide or encase themselves in protective material. To add dramatic effect, apparently, the Sword makes a "howling" sound as it flies, and tends to be accompanied and/or trailed by a streak of "spectral" light (redshift/blueshift?). Farslayer also "phases" through matter, becoming solid only upon entering the body of the target. Essentially, the Sword brings instant death for the victim; instant revenge for the releaser.
Ah, but therein lies the catch! The Sword does not return -- this is a godly Dagger, not a Boomerang. Once you have thrown it, you have also handed it to someone else... the first to reach the body. If they can guess who has thrown it, and they wish revenge for the act, they, too, may then hurl it out of sight, possibly back to the original revenge-seeker.
We see this scenario played out to its extreme in The Fourth Book of Lost Swords: Farslayer's Story, one of the better of the Books of Lost Swords. Of course, the first to find the newly-thrown Farslayer may not wish revenge for the act, or may seek to strike elsewhere. Every throw of the Sword is a gamble -- but one that pretty much guarantees at least a partial victory through its vengeful result, with only a few exceptions:
All other Swords seem useless against the Sword of Vengeance -- potentially, Wayfinder could lead you to somewhere you might find something useful to protect yourself (another Sword), or a way to remove or dissuade the person who wants your heart as a shishkabob... if you had enough time before the Sword was thrown (say, if they had a lot of people to choose from). Most likely Wayfinder would respond with the Sword equivalent of a blank stare if you asked it where you could go to get away from Farslayer -- especially once the Blade has been cast. You know the song: "Ain't no mountain high enough; ain't no valley low enough; ain't no river wide enough, to keep me from you".