I disagree. It appears you think I am implying a railroad or something.
Incarnated power of plot devices don't merely imply a railroad, they are a railroad.
If you have a device out there with an absolute power which has no saving throw, then its entirely a matter of DM choice what the players (must) do. This is especially true of items where the absolute power no saving throw includes things like mind control.
This is a game world where a PC's can simply be scrubbed out because suddenly Farslayer appears in ones chest. Items like Farslayer always favor the NPCs for the simple matter that they deprotagonize everyone. This is a game where if the DM decides some NPC has Mindbender or Soulcutter, it's an automatic TPK. What prevents these occurrences? Not even logic; only the DM's predetermined plot and events.
One of the main themes of the books is that in the game, no one - not even the gods - is actually in control. Events and destiny just sweep everyone along with no real power over their fortunes. Everyone makes choices because they don't have a choice. They have only one path available to them. The sword becomes the only choice, and once the sword is drawn its making the choices for you.
Heck they could decide to destroy them all.
And this would differ from the narrative of the books because...?
No, the more likely scenario IMO is that unlike the books, all the swords don't end up destroyed. The PC's differ from the text in that basically they will play well together (unless the DM chooses to force them to kill each other, they'll game the situation). Basically, the DM must either choose to kill the party in the manner of his design, or else choose to let the party collect swords until they become invincible. The only mystery here is how poor the DM is at predicting the outcome of his own actions.
UPDATE: I think I can explain this position more succinctly.
In every setting there are potential foes of high challenge rating. The DM could in fact challenge the PC's at 1st level with elder dragons, fiend lords, and storm giants taking a very personal interest in their affairs. In such a situation, the PC's would be completely subject to the whims of the NPC's - and thus perforce completely subject to the whims of the DM. Thus, in general, DMs refrain from having high challenge NPC's taking personal interest in the PC's (at least in a hostile way) before the PC's are sufficiently high level to thwart the will of the NPC.
But what is the CR of an NPC like Farslayer, Mindbender, Soulcutter, or something else of the sort that is invincible with no save, and no spell resistance, and trumps even immunity no matter who you are - even a CR 60 god? The CR of such an item is infinite. It has the power of plot. It's appearance determines the outcome. As soon as it shows up, the PC's are completely subject to the whims of the item and its wielder and thus perforce completely subject to the whims of the DM. Thus, in general, a DM should always refrain from having such objects part of his game.