The Skull of Shadows is an ancient artefact, the head of the Seventh Dragon. A long time ago Sar-Elam sacrificed himself to save Ashan from demons. The Skull of Shadows is the only thing that can fully release demons from their prison in Sheogh.
Many searched for it, like the mage Menelag, but it wasn't until Sareth, the Dark Messiah himself, that it was found.