The doomsayer is usually portrayed as an old man. In reality, there were oracles and witches and voodoomen about that had a secret knowledge that people required. In dreams, there is the anima/animus figure, in men represented by an old woman shrouded in darkness. That is in Jungian terms, age for wisdom and darkness for how the truth of that figure is obscured to us, the viewer.
In the previously mentioned Wes Craven films(Last House On The Left, Hills Have Eyes, and Nightmare On Elm Street), there is a doomsayer only in Last House On The Left. He had a sense of exactly what was going to happen, and he was heading for the hills himself, but only too late.
Friday the 13th had the figure of the aimless Ralph, a somewhat nutty old man on a bicycle. His wisdom may have been nullified by circumstance. But he was right nonetheless, no matter how crummy his clothes or bicycle were, no matter how inarticulate his speech. Those people were doomed, just like he said.
Like he said to their startled faces.
Perhaps the greatest of the prophetic Doomsayers, for me as a horror fan, is Doctor Samuel Loomis in Halloween, as portrayed by Donald Pleasance. He knew what was happening, bought a gun, and went in and got his hands dirty cooling out the situation. Now one can argue, that by injecting himself as the hero in the end, that he is not a doomsayer, but a hero, like a lawman of an old cowboy epic on the trail of the villian.
But the moviegoing public and John Carpenter kept Jamie Lee Curtis as the center of that universe, that she is the hero for even surviving the horror.
Keeps the plot going. Got a lull? Toss in a cryptic warning that eats at the edges of the bright sunlight of day: the night is coming; make ready yourself for doom. This was the exactitude of Friday the 13th, though there were early kills, too, but this contributed to perhaps the solidity of the mass of the film, as an extra element in the sauce of summer camp story: that we never forget and feel at least a sense of dread, if not outright Spider-sense tingles.