Observing the effects of the trap might tell us something- there are certainly enough affected people on the trail that need to be helped. [The notion of using herself as bait has not yet occurred to Sabriel. But Sabriel still isn't convinced that this isn't some sort of elaborate spell.]

However, if this is a spell, rather than a spirit... either it was meant to do something else, but degraded over time, or something was placed at the end of the trail once, but is since long gone. Or perhaps it was meant as a trial of sorts. [A trial makes more sense than a guard spell, to Sabriel. Something to test ones ability to resist temptation, or follow instructions? And was it placed by the same force... or entities that had brought them here?]