The Duller, classified as Entity 6 in Backrooms lore, is a tall grey humanoid with no facial features and no visible sensory organs. Its most striking trait is a pair of long arms that extend over great distances; avoiding direct confrontation, it reaches those arms through walls or floors to snatch prey and drag it away.
The heart of the horror is a reach that nullifies every boundary. The instant you take comfort in a single wall between you and it, an arm extending through that wall collapses all your assumptions of safety. As a threat that is not seen but can reach you from anywhere, it denies the reassurance of distance and cover.