Personal key of Walgrave
The personal key of Walgrave is a key that can supposedly open every door at the Citadel. Only archmaesters possess such keys. Old senile archmaester Walgrave's key is stolen by his young aide Pate, and ends up in the possession of the Alchemist - a suspected Faceless Man, possibly Jaqen H'ghar.
The key is old and heavy, and made of black iron. Other archmaesters carry their keys upon their person or hide them away in some safe place; however Walgrave had hidden his in a stout strongbox underneath his bed. The strongbox's lock was broken by Walgrave after he lost the key that opened it, making it easy for Pate to access the key.
A Feast for Crows
Pate steals the personal key of Walgrave, in exchange for a golden dragon from a person who calls himself the Alchemist. Pate's steals the key due to needing a gold coin to claim the maidenhood of his beloved Rosey, the youngest serving girl of the Quill and Tankard inn.
Before Pate hands over the key in a narrow alley in Oldtown, something makes him hesitate, and he asks what the man wants with the key, as some of the old Valyrian scrolls down in the locked vaults were said to be the only surviving copies in the world. The Alchemist replies that what he wants is none of Pate's concern.