Rilthus stumbled through the underbrush. Banished from his town for barely anything, just a little bit of necromancy, honestly, he was going to give it up soon anyway (he was kind of bad at it), he had somehow gotten lost in this forest. He had stepped off the road for but a moment to simply urinate, and somehow gotten lost.
As he trudged onward, he curse the town that had banished him, and had, however unwittingly, forced him into this miserable, wet, and mosquito-infested wilderness. He would get them, oh he would. He would get his vengeance.
The forest broke after thinking these thoughts, and before his eyes he saw a hill, lacking a single tree. And at the top stood a circle of stones, more like boulders. Rilthus staggered up the hill, entered the circle, and had his mind blown.
The Circle of Culthus is impossible to find- at first. The average person could wander the world forever without discovering it- not that they would know about it in the first place. Until it is discovered, the Circle is unfindable and unknowable except to that one special person.
That person could be anyone. All they need is a little bit of ambition for something. Perhaps its revenge. Maybe their hungry for power. But somehow, what they want is undeniable to them. They lack the skillset or the funds to enact their plans. The Circle will pick that person.
They will be walking around in the area, and for some reason, decide to head off into the wilderness. They might need to take a leak, or saw some fruit, or something of the sort. Then they get lost, and wander about towards the Circle.
The Circle of Culthus itself is a simple collection of boulders surrounding a stone pedestal. The boulders, if one takes the time to look, are perfectly placed in a perfect circle. And the positioning of the boulders is perfect too, with one at the exact north point, one at the exact northeast point, and so on for the rest of the six cardinal points. The stone pedestal itself is at the exact center.
The person who finds this place will be awed at their discovery. They hadn't heard of this place. What luck. Perhaps they aren't so pitiful after all. And that is when the Circle begins its work.
Rilthus, as he sat for a moment, leaning against a boulder, pondered his discovery. A stone circle, previously undiscovered. What could he do with this? And in a voice that wasn't quite his own, whispered into his mind, perhaps... perhaps if I were to start a religion here, a religion that preached death and worshiped that pedestal, I could gain enough power to get revenge on my town. I know a bit of necromancy, perhaps I could trick some people into joining me.
He stood, and considered his plans. Yes, yes, a cult. That could work. A cult...
Perhaps it was the way the sun emerged from behind the clouds exactly when he though “a cult,” or perhaps it was the air of these old stones, but Rilthus's undertaking seemed to be blessed. By who, Rilthus had no clue. But he had to thank them, as he drove the dagger into his human sacrifice a month later upon the stone pedestal in front of his congregation, this cult thing is working.
Somehow, the person who comes to the Circle of Culthus will get an idea to start a cult. And they not only think of it, but they carry it out, too. At first, their actions gain them lots of followers. People seem happy to join in. And then, when the person becomes the priest, the focal point being the stone pedestal (which has somehow stopped looking like a pedestal and started looking like an alter, and when exactly did these little blood-drainage channels appear?), the persons ambition is fulfilled. Somehow or the other, they succeed.
From that point on, the person is the Circle's. That person is Culthus's. For ages ago, long before any cult touched these stones, a group of mages summoned the demon Culthus, to serve as labor in a civil service for their city. To make a long story short, Culthus broke free of his bindings, and ran rampant over the countryside. Unable to return home, he decided to stop as many demon summonings as he could, to benefit his brethren.
Culthus was eventually bound into the stone circle's pedestal, and has found out how to escape: belief, and human blood. Animal helps, but the blood needs to be from some sentient organism (humans, elves, dwarves, goblins, and orcs) to have the most effect. And thus, Culthus the demon uses his influence to get humans to form cults and sacrifice upon his chains.
And when the eyes of the world turn to the Circle of Culthus, and eradicate this next cult that pops up here, the cult dies off easily. Perhaps too easily. For Culthus knows there is always the next human to manipulate. And to help that manipulation, he makes the circle vanish from the history books and people's minds, returns it to is unfindable state, and waits.