Savilla's Stone is a special stone used for scrying, kept by the Order of the Ancestor Moths . In the Thieves Guild quest, Turning a Blind Eye, you steal the stone from the Temple of the Ancestor Moths. The Gray Fox uses Savilla's Stone to find the items needed to complete the Ultimate Heist.

When you become the Gray Fox, you can find the "Savilla's Stone" in the new guild hideout in the Waterfront of the Imperial City, (the new Guild hideout is created after you become the Gray Fox).