It is owned by Nero the troll, and was given to him by the demon Graulas. It allows him to see most areas of the Fablehaven preserve.
Fablehaven
Seth, Kendra, and Ruth need to find Grandpa and Lena, so they go to Nero to use his seeing stone for help. Nero agrees to help in exchange for a massage from Grandma. He later tells them that Lena and Grandpa are in the Forgoten Chapel.
Grip of the Shadow Plague
Grandma, Grandpa, Dale, and Hugo go to Nero to find out more about the figure that was released from the Quiet Box, but find Nero collapsed under a log, with his seeing stone gone.