Prison Island was the smallest of Riven's five islands. After the Age broke up into different land masses, this island drifted away from the other four until it could no longer be seen from any of them. Isolated as it was, the only way to access it was via a Linking Book from Age 233.
The island was dominated by the stump of a massive tree. This "great tree" was once the tallest in all of Riven by a wide margin, but Gehn had it chopped down and turned into paper for his Linking Books after he was imprisoned on Riven by his son, Atrus, and a Rivenese native named Catherine. He later built a small structure on top of the stump and converted the island into a remote prison. When Catherine was tricked into returned to Riven in 9461 DE, she was eventually captured and imprisoned on this island until the Stranger was able to set her free.