You beat an Arch Demon, you freed Yurt who kills everyone in Nexus, or Mephistopheles is there. Probably just that you beat an Arch Demon though. Dragon God is usually the first arch demon you beat. Others are Allant and Maiden Astrea if I'm correct.
Beating an Archdemon doesn't do it. I've beaten storm king and no new music. --- Falwell on O'Connor:"Every good Christian should be concerned" Goldwater:''I think every good Christian ought to kick Falwell right in the ass.''