The Savilla's Stone is a special stone used for scrying, kept by the Cult of the Ancestor Moth. In the Thieves Guild quest "Turning a Blind Eye," the Hero is tasked with stealing 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 the Hero becomes the Gray Fox, they can find the "Savilla's Stone" in the new guild hideout in the Waterfront of the Imperial City. It has then stopped being a quest item and has a new ID.