Humans are far from the first form of intelligence to inhabit the Earth. Before man came the spirit and the beast; before those primeval kinds came more alien beings that colonized the primordial Earth from distant stars. Some ruled over the planet for millions of years and then died out in the great convulsions between ages, leaving behind only the most puzzling ruins. Some retreated in the face of enemy races; others left of their own accord. Others linger in the desolate places of the Earth, still threatening to the comparatively young and primitive race of humankind.
The starspawn are extraterrestrials, and their aged presence in Thule and the nearby lands aren't yet gone in quietus. Of the identifiable few, these are the abominable mi-go, the fungi from Yuggoth; the voracious moon-beasts, blasphemous horrors from the realms of dream; the night-gaunts, blind servitors of nightmarish entities; shoggoths, huge and amorphous; the stranglers of Nheb, who carry their victims out of the atmosphere altogether to choke them; and the star-less armadas of the Dark Tapestry, who fear none but the dawn star's light off the mirror towers of Kal-Zinan.
These unearthly beings are not necessarily supernatural, but their alien origins mean that they are often armed with powers beyond the comprehension of Earth's native inhabitants. Due to their sheer unpredictability, starspawn can be extremely dangerous when hostile. Even the careless Atlanteans—who are arguably starspawn themselves—fear them to a degree that necessitates deep care and research.