Key to Shadow
From Diablo Wiki
The Key to Light and the Key to Shadow were two magical and mystical gems fashioned by Lord Juris Khan with the help of his daughter Atanna in the novel Diablo: The Kingdom of Shadow written by Richard A. Knaak.
The purpose of the Key to Light and the Key to Shadow was to anchor Ureh in the real world.
The Key to Shadow was a raven-black twin placed in the deepest of caverns, far beneath the palace of Ureh.
At the end of the story, Captain Kentril Dumon shattered the Key to Light by dropping it to the ground from far above. It shattered into many pieces.
However, it is never revealed what happened to the Key to Shadow. Zayl went to the caverns below to destroy it, but we never see him destroying it. The next time Zayl is shown, he was attending the unconscious Kentril.