The Old One

The Ancient Kingdom of Land's End has existed since (apparently) the dawn of time. How it was formed is still under wraps by some of its oldest members, but the general idea was that when the world was created and the land formed the ancestors of the land were cultist. The first of their kind too, while most didn’t believe or do magic, they wholeheartedly believed it and strived for a cure to mortality. While strange to others, they were secluded and often didn’t interact with others, remaining by themselves and deciding it was the best course of action – what they worshiped was an entity. One that none knew existed, only thought to exist. A monster-like entity that only lurked around when day turned to night, or when somebody wasn’t with others. Reports of missing people, bloodshed, and crude displays of murders were rampant and the surrounding things among them weren’t as capable.
And so? The cultists believed this to be a beast that watched over them, but required sacrifices to keep sated, live sacrifices first believed to be required to be virgins, strong men, women, or children. Anything that was young, or youthful. For it lived forever and lacked youth, so it wanted to feel that once again. Some believed it tortured, others believed those that died did so in the most pleasure they’ve ever felt in their lives. It varies and the story builds and builds, but readily gets corrected as time passes.
As time passed it got worse as well, the dawn of civilization found the cultists performing their own rituals in order to connect with the creature or creatures. They sacrificed many of their own, most attempting to somehow find immortality through these means. Hoping that the creature only exited off the belief of others and through the rituals they’d be able to evolve into it – this wasn’t wrong or right per se. However, at the start they found no success, they only ended with a pressing civil war on their own hands and while the cultist generally outweighed the general public – not all cultists agreed with each other’s way of handling things.
They split up, fighting among one another, and nearly wiping themselves out as a result.
Luckily! Not for them, however, the creature of legend did eventually arrive. On the brink of extinction, with the groups at each other's necks, the group that believed in the rituals was about to be felled by the union of the other two groups. Their army was almost destroyed, and their culture almost redone completely.
That is when “The Old One” arrived, in a blurred movement during the darkness of night it did come. Those rose up arms – while mostly stone weapons and clubs, but alas, they tried. They were felled in what seemed to be an instant, for the beast was unlike those it was attacking, it aimed for spots none would ever think of hitting. It knew the art of murder like the back of its hand an by the time it arrived, it had already calmed the rage of war down – letting fear settle in.
Fear and excitement for most, typically. It spoke no words, but its voice could be heard among all, not loud nor booming. Only calm and soothing, cooing to them in an almost unnatural way to drop their weapons and so they did. What happened next is what is considered to be the birth of the “Firsts” and the “Realms”. The appearance of the Old One is stated to vary depending on which family or realm tells it. Some recount that it had red hair with blue and black eyes, or blue hair with black and red eyes. Most say that it had black hair with one red and one blue eye. Its gaze was terrifying, its presence almost impossible to move out of.

Summary

The Old One is an ancient, godlike human-like thing that defends it's homeland, only due to the faithful cultist praying and worshiping it for as long as time lived. The myth to others is that it doesn't exist and they are crazed cultists, the stories vary from person to person once you get further and further away from the capital.

Spread

Despite being a 'myth' this is only remembered by those that are thousands of years old. Or any civilizations that are for that matter.