In a distant corner of the cosmos, an ancient white dwarf star is orbited by a lone world: the mysterious planet of Carthoria. All formal surveys and studies reveal the same thing: it is an empty world of vast, cracked plains and jagged mountain peaks that shouldn't even have an atmosphere but somehow maintains thin, cold air and everpresent banks of fog, all in a cool pastel color palette that blends together and makes discerning details or figuring out where you are increasingly difficult over time. Utterly silent, there are no indications that the world has ever supported any kind of life.
And yet, those who dare to spend time wandering Carthoria claim that they begin to notice things. Unnatural shapes moving across the plains that disappear if you try to focus on them. High-pitched buzzing sounds with no discernable source that make sleep impossible. Hazy buildings that appear and disappear in the mist whose geometry hurts to look at, and vivid dreams of statues with indescribable faces that have them waking up in psyche-scarring terror. Shadows deeper than pitch black that blot out the stars.
In some of the dirtiest, most backwater drinking holes in the galaxy, determined researchers can find grizzled old spacers who claim that there have been multiple attempts to build settlements on Carthoria, but they have all disappeared from both space-time and memory.
Serious scholars and reputable astronomical organizations dismiss all of this as the overactive imaginations of long-haul travellers addled on mind-affecting drugs, and no one has been able to prove otherwise.