The Doom that Came to Sarnath
Welcome to the world of Sarnath, where shit is about to get real.
Sarnath is a world with high-fantasy elements but low prevalence of magic; that is, magic is real but uncommon. The PCs are exceptional characters, and therefore may be more predisposed to the use of magic than the average commoner. The PCs start their campaign in a feudal kingdom dominated by the race of men, with institutionalized halfling slavery. Again, the PCs are exceptional, and need not be humans just because they start in an area populated with humans.
Most campaigns focus on the struggle between good and evil. Pathfinder's (and D&D's) alignment system offers a much less frequently explored struggle between law and chaos, and this campaign will focus on the latter. Here we see the effects of chaos in its most maddening form, inimical not only to biological life, but to creation and consciousness as we know them. Inspired by the works of HP Lovecraft, horrors have begun with increasing frequency to slouch into Sarnath. The PCs begin play unaware of this fact, but will quickly be made aware in an unwelcome way.
The intent of the campaign is for the PCs to collaboratively triage these horrors while seeking to identify the cause of their appearance and end that cause. Their personal motivations for doing so are their own, and all PC alignments (including chaotic alignments) are welcome as long as they can work together for this common task. PCs may begin play knowing or not knowing one another, but regardless, players are encouraged to collaborate when creating characters in order to ensure that PCs will be able to work together with minimal infighting.
Finally, players should be informed vaguely of the nature of their enemies so that character builds can be planned appropriately. Regarding alignment, enemies may be of any alignment, but with a moderate bias toward chaotic enemies. Because it is in chaos' nature to be unpredictable and have unpredictable effects, it should be noted that some horrors, though born of chaos or allowed entry into Sarnath by it, may be lawful. The important thing is that such beings are still alien to Sarnath and pose threats to it. Lawful does not mean friendly. Still, the bias will be toward chaotic enemies.
Regarding creature type, the bias will be toward aberrations and plants. Aberrations are truly bizarre beings with anatomies unlike those of other beings found in Sarnath, usually due to an alien origin. Some small number of native species, such as the disgusting otyugh, are classified as aberrations just because they represent a small and very different evolutionary path, or perhaps mutation by toxin or magic. HP Lovecraft's works and those inspired by him involve a non-trivial number of fungus-based lifeforms, which Pathfinder includes in the plant type even though in reality plants and fungi are two separate kingdoms. Of course, Lovecraft's works also include plenty of crazed humans and no small number of spirits, so humanoids, outsiders, and undead should also be expected.