From the "Legion: Warlock Artifact Review" official blog: Before the eredar served the titan Sargeras, one of their greatest leaders, Thal'kiel, had unprecedented skill with summoning and binding magics. Driven by ambition, he reached into the Void and was answered with knowledge of dark creatures unlike any the eredar had seen before. Thal’kiel’s apprentice, Archimonde, discovered his master’s dark pact and revealed it to Thal'kiel's fellow rulers, who banded together to strike Thal'kiel down. After his defeat, they had his skull gilded and placed on display as a warning. Today the dread lord Mephistroth uses it to enhance his ability to summon and command demon armies for the Legion.