The mechanism behind the appearance of demonic hordes is still unknown to even the best minds of the world, but this does not prevent the prediction of such catastrophic events. Throughout history, several seers have been known to be able to accurately foretell the timing of demonic invasions; almost all these prophets are elves, or of elven blood. This is not to suggest that the elves possess some special quality that affords them particular prescience; rather, this is simply an result of elven lifespans, the immediate nature of predictions, and the frequency of demonic appearances.
With a considerably large margin of variation, demonic invasions tend to occur about once or twice a century on average. Therefore, most of the free and good peoples of the land, humans in particular, would only experience one or at most two demon invasion events in their entire lifespans. On the other hand, even the best of divinations is only able to foretell an event a decade in advance. The combination of these two factors means that it is very difficult to assess the accuracy of would-be seers specializing in predicting demonic invasions. A single correct prediction could easily be a fluke of chance; a second correct prediction made fifty years later would provide more convincing evidence of the seer's prescience. However, considering the lifespan of a typical human, it is very unlikely that a confirmed seer would live to be able to make a third forecast. An elf, or on occasion a half-elf, would not suffer from this problem; their greater lifespan would even allow more opportunities to confirm their predictive efficacy.