If I were to tell you that in Pathfinder a) spontaneous human combustion is a real thing, and b) that there is an undead creature created by the phenomenon, you would quite frankly c) probably say that is dumb.
So how about I tell you that the combusted is an always-on-fire skeleton that is perennially screaming with a stunning Howl of Agony (Ex), that it hurls fire like a kineticist, and that its slam attacks carry a curse that actually sets PCs on fire…and if reduced to below 0 hit points, they die instantly of combustion too? How’s that sound?
Yeah, that’s right—it sounds awesome. Because it is.
Besides, while SHC is a myth and a joke in our world, in a fantasy setting plagued by wild magic, haunts, and psychic upwellings, it's a very real potential threat, however rare. And while magical or psychically induced combustion may be rare, it makes sense that the victim’s horror and pain would be enough to spark an undead rebirth. In fact, you won't find this text in the link above, but the full entry from Occult Bestiary features two locations in Golarion where these creatures are common, and for good reason.
And here’s one more reason to use combusted. Say you’ve never had psychic magic/occult/psionic rules in your campaign before, and you want to introduce Occult Adventures. You can hand-wave it and say that psychic classes and monsters have been there all the time and the party just didn’t notice them. Or you can have them be the product of a far distant land or alien influence (or both, like Eberron’s Kalashtar/Quori from Sarlona). Or you can make the advent of psychic magic in your world an event—something game-changing that happened to your campaign. Such a violent upwelling could have all kinds of crazy effects—included setting fire to many poor mortals right where they stand, leaving only undead horrors behind. Then for the rest of the campaign, combusted are one more sign of the new world the PCs must contend with. (Which is bad enough on its own, but it’s even worse if the psychic eruption was because of something the PCs did…or failed to do…)
Dishonor is considered a fate worse than death in Sapphora…but for the Mikhal clan it is the literal truth. The Mikhal family labors under an ancient curse: If one of their number dies with a stain on his honor, he reincarnates as a combusted—a fiery monster screaming in agony that then attempts slay all who encounter it. Desperate to avoid this fate, Mikhal youths often pursue callings as samurai, paladins, and priests (all careers that afford them clear codes of conduct and ready paths toward absolution). They will also accept any duel, answer any challenge, and otherwise go to any lengths to keep their honor intact—no matter the cost.
The lesser wendigos of the Kaltan Reaches are not the wind walking, dream haunting nightmares that plague other arctic realms. Nevertheless, they are still terrifying hunters. Roughly resembling flying, elk-headed bugbears, lesser wendigos lure their prey away from the safety of the longhouse, then chase them through the woods until their victims literally burn away in fear and exhaustion. Many of these victims never stop running or screaming, transforming into undead combusted on the spot. The lesser wendigo loses interest at that moment, leaving the undead to haunt the snowy woods in eternal fiery agony.
The blue-skinned, gray-haired Merovians are blessed with an elemental connection to the Plane of Air (treat as sylphs). Indeed, the fact that they carry their own breezes with them is all that allows them to survive on their isolated, nearly airless moon. But their oxygen-rich natures make them vulnerable in fire-rich environments. In fact, they cannot enter certain sun-filled crystal spheres or risk bursting into flames on the spot. The trauma of their combustion leaves only the undead husk of the Merovian…but that fiery husk is often more than enough to murder every hand aboard a voidship.
—Occult Bestiary 16
What’s a group of combusted called? That’s right: an effigy.
Why yes, I did have the Ray Charles version of “Busted” playing in my head the whole time I wrote this.
Speaking of songs, I had a suitemate who loved “Spontaneous Human Combustion” by the Bobs. Like, way too much. I feel like my life has been leading up to this link.