Part of the Casswiki article series Cassiopaean Experiment and Channeling and channeled material
A psychomantium is a specially set up darkened room used for skrying, or seeing spirits or other things outside of the normal sensory plane through the use of a mirror.
The medium sits before the mirror, which is slanted so that it will only show the roof of the room, which is black, as all else in the room. Candles are placed behind the mirror so that there is minimal indirect lighting reflected from the mostly absorbing black walls. The medium may then see things in the mirror and use this as a sort of communication or viewing device.