Edit: Sorry this is late. I caught a summer cold that knocked me for a loop last week, and Friday night I coma-ed out after work so hard I didn’t even get a placeholder post up.
A famous Lovecraftian creation, Yog-Sothoth in its Pathfinder incarnation is an Outer God from beyond the stars, according to Bestiary 4. In fact, it might even be the void between the stars, because that’s just how Yog-Sothoth rolls. But every so often it’s nice enough to take time off from being an unknowable horror in order to impregnate some human so she can give birth to a loathsome spawn of Yog-Sothoth. That’s just the kind of giving alien entity Yog is.
The spawn of Yog-Sothoth are naturally invisible, smell to the high heavens (seriously, you have to save against it), and are basically squirming masses of awful. They’re also geniuses and often take oracle and witch levels. How that genius manifests is hard to pin down from a mortal perspective, since they’re basically nonstop ravenous killing machines. So don't expect an edifying conversation from one (even assuming you can/dare to speak Aklo), but do expect that it will make wise tactical decisions. And if you try to hide behind the puzzle-locked door of some dungeon chamber, it’ll just solve the puzzle in a flash of alien logic…or smash the door to flinders.
And as with most horrors from beyond this reality, spawn of Yog-Sothoth rarely just happen. Somewhere along the way, a cult was involved, a blasphemy performed, an Outer God invoked…so if you run into one of these spawn, expect to also run into whoever called it or whatever dark fallout accompanied its birth.
Lady Marchand has never been quite right, not since childhood—since the deaths of her sister, who fell prey to an attic whisperer, and her brother, who was carried off by the yellow pox. She grew up a lonely and suspicious young woman, though her good looks and wealth guaranteed that she still had her pick of husbands. After her first pregnancy ended in a miscarriage, she was determined to have a babe that would be “strong enough for this cruel world, and all the worlds beyond it.” It’s unclear when she fell under the sway of the Black Gate or offered her body to Yog-Sothoth. But dark things move through her manor now, will-o’-wisps circle in strings around the moat, and her misshapen but truly strong child slurps out over the drawbridge on overcast nights to feed…and grow.
On the night of the syzygy, adventurers race to align a giant’s magical telescope with one of the planets. If they do so, they will form a link to that planet—one that could teach them many things, or perhaps even serve as a gate. Of course, they are not the only ones in the observatory this night. The local lord’s warmages want the telescope aimed at a nearby star whose power they can tap to fuel a doomsday weapon. Meanwhile, a cult of mad prophets wants the telescope to remain where it is, pointed at the void of space. If the telescope is pointed at nothing but blackness when the planets align, they believe the blackness will send its scion to bless them: a spawn of Yog-Sothoth.
Adventurers come across a room stained with dark matter. A little later in a completely different part of the dungeon they come across the same room, only this time spotless but now filled with an invisible but disgusting horror. If they defeat it, they recognize the crusty residue left behind forms the same stains they saw earlier. But how could they have already seen the stains of a battle they had not yet fought in another room entirely? Have they become unstuck in time and/or space, and if so, how? Worse yet, if they don’t defeat the spawn of Yog-Sothoth in that room, the resulting paradox begins to unravel their connection to space and time, shunting them into a nearby dimension where the spawn of far worse creatures lurk.
—Carrion Hill 28 & Pathfinder Bestiary 4 251