The unknown is unseeable by humans except for crimson eyes and a vibrating tail of richest black slime. This thing is untouchable but it could grasp you by the shoulders with obsidian claws, pushing you to the ground. The unknown is called the unknown because it cannot be studied, it cannot be contained. That is why people represent it with a symbol that looks like this:
It feeds on fear so run and hide when the beast is near. Those who are without hope stay calm and give themselves up; they know there’s no escape whatsoever. They have read about this shadowless beast (with its rating of 100 in the Deathopaedia Book 2). It waits until they are scared, then it stares at them to make them freeze in fear. Then it imprints the symbol on their wrist. Only then can it feast on their fear.
As the terror of the unknown does its work, the echo of painful screams grows fainter until they are heard no more. So if you get this symbol you are not alone. Others have gone before you to their horrible death.