A small piece of broken skullbone
There are 16 of these presumes to exist, speculated that there might be more. When the flayed god Aoskar was ejected from Sigil, he fell somewhere and his skull was partly shatrtered and 16 pieces landed on each gate-town on the outlands. Some have been recovered, others have been lost, others are still being contented for.
The more items a person possesses, the stronger the link he has with the whispers and the visions of the dead (or not so dead) god that lies somewhere on the planes. It is unknown what will occur when all pieces have been bound together.
Effects of several pieces being in proximity on the same person
1 piece: Faint whispers of the dead god, unintelligible.
2 pieces: Sudden flash of a skull lying in a barren wasteland, happens whenever one sleeps.
3 pieces: Vision of Aoskar rising towards Sigil and setting it straight, while the Lady appears.