In the center of the chamber, a large, beruned millstone grinds a deep channel into a floating moebius strip of faintly purplish stone. There is no clear mechanism by which either the millstone or moebius strip are suspended.
MADNESS OF THE MILLSTONE: 1d8+2 rounds after entering this chamber, characters must make a Will save (DC 15) or suffer 1d3 points of Wisdom damage. As Wisdom damage is suffered, the runes on the millstone begin to glow faintly blue and its motion along the moebius strip speeds up.
The save is repeated every (10 rounds – 1 per 5 points of total Wisdom damage the millstone has inflicted). The faster the millstone goes, the deeper the groove it wears in your mind.
(This material is covered by the OGL. See attached PDF.)