Is literally a chunk of nothing/void/nonexistence. It ‘infects’ every living thing that touches it, and every time it shifts realities/worlds the infected things follow.
The stone is about the size of a closed fist (is there any other kind of fist?) and feels temperature neutral to the touch. It has no weight, no smell, no taste, and has touch by the absence of sensation. Looking at it tends to give one headaches since there is literally no input from the space it occupies. The brain, helpfully, tries to fill in the gap and thus everyone sees it a bit differently. Most people see some sort of darkness, but often there are light artifacts. There is a feeling of having gone ‘blind’ in the area the stone occupies.