A scrying glass, and a beacon
The Shining Trapezohedron is a chunk of unearthly black stone carved into an unusual shape. It can be used to gaze upon this and other worlds, but the things in those worlds sometimes gaze back.
A spell in the last chapter of De Vermis Mysteriis requires possession of the Shining Trapezohedron to cast successfully. The spell will call forth the Haunter of the Dark, a destroyer of worlds.
The stone has unusual qualities. It fluoresces in the dark. It hovers as though weightless. To touch it can cause a discharge of ethereal energy potent enough to char flesh.