the evil that is real for you alone — "no one else in the room…"
no one else in the room feels the presence of something radically evil; none thrash or sit damply up; no one else cries out: whatever it is is not evil for them.
Context
The child's first night away from home delivers the pure form of the unshareable: a total evil overwhelming and undeniable to him, occupying the same room as three sleeping boys for whom it simply is not there. The horror is not only the leering face in the floor but that its reality is sealed inside one skull, felt by no one else and impossible to transmit. And when the mute face finally acts, it only opens its mouth at the light, a silent thing that cannot be said and cannot be shown.