Shadowvapor is similar to Descrying potential, in a simpler version. It is a mental substance only able to be sensed by Shades and it is used to see honesty and trust. Shadowvapor is a blocking for one's secrets, and illumination is the opposite. By reading the shadowvapor, you are technically reading the potential of the darkness you have. According to Tam and Linh Song, the two cancel each other out. When a Shade reads someone's Shadowvapor, it feels like a blizzard blasting through their head. To read Shadowvapor, a Shade will move their shadow over the person, similarly to Shadow-whispering, and perform a reading. Sophie, Fitz, Biana, Dex, and Mr. Forkle all had their Shadowvapor read by Tam Song, in Book 4: Neverseen, while Keefe and Alvar refused since they were hiding something.