"The God called Stone," he says. "My sister by marriage. Of all of us, she is the best."
The Mountain of Light is many things: a living mountain far to the south of the Presbyterate, a zee-god called Stone, the source of the Wax Wind, the daughter of the Bazaar and the Sun, the mother of Mt. Nomad. People from around the Elder Continent regularly make pilgrimages to her, and sometimes they clip relics (called Mountain-Sherds) from her slopes.
The Thief-of-Faces - the original Snuffer - wounded the Mountain of Light long ago and made her child, Mt. Nomad. The blood from this wound forms the Nameless River, whose estuary into the Unterzee is called Adam's Way.
The Mountain is likely named after the diamond known as the Koh-i-Noor (Mountain of Light in Persian). This diamond was once the largest in the world, and passed through the hands of the Kakatiya dynasty in India, the Persian Empire, and the British Empire. It is now a part of Queen Elizabeth II's crown.