The crystal skull represented the cranium of an extraterrestrial alien being from another dimension. A Trandoshan hunter named Garnac possessed one of these skulls, which he stored aboard his Ubrikkian Floating Fortress situated on the moon called Wasskah.
The Indiana Jones film, the Kingdom of the Crystal Skull, released in 2008 and also brought to life by George Lucas, heavily featured the crystal skull. The in-universe skull replicates the design seen in that cinematic portrayal.