The Thief Stone

Stealth skills — Alchemy, Light Armor, Lockpicking, Pickpocket, Sneak and Speech — improve 20% faster.

Activating a standing stone grants its blessing permanently, but you can only have one stone's blessing active at a time — activating another simply replaces it, at no cost. The Warrior, Mage and Thief Guardian Stones near Riverwood are usually the first ones you reach.