The Crown of the Skeleton King
From Diablo Wiki
The Crown of the Skeleton King is a quest item the player must secure in order to complete The Skeleton King quest. See that page for more details.
Blizzcon 2008 Demo
In the 2008 Blizzcon demo, the crown was obtained on the second level of the dungeon from the leader of a group of Dark Cultists. After securing the crown, players had to venture down to the third level of the dungeon and fight their way through to Leoric's throneroom, where the ancient King's moldering bones were perched on a throne.
Delivering the Crown caused King Leoric to reanimate, triggering the final boss battle that concluded the Blizzcon 2008 demo.
Leoric's reanimation and the subsequent battle can be seen from just after the Crown was used to awaken him, in the following YouTube video.