SCP-106 is a humanoid in the form of a decaying old man. Objects and surfaces it touches corrode and blacken, and it can phase through walls or floors to appear suddenly from anywhere. Those it seizes are dragged into the dank pocket dimension it inhabits and tormented there as if hunted.
The heart of the horror is the abolition of safe distance. Neither walls nor distance offer defense, and once you are marked there is nowhere to escape to. Where physical containment is rendered powerless, it lays bare how thin the belief in controllability really is.