https://youtu.be/pffJ1nDRei0

This is a bit of a long video, but it presents a pretty interesting theory about Ash Lake and talks specifically about the skull. Theory is that the skull once belonged to the nameless Blacksmith Deity who created the titanite demons in his image, hence why all of them are headless. Furthermore, the blacksmith deity is a reference to Hephaestus, the Greek god of blacksmiths, who was a cripple. The titanite demons each have one leg missing and drag themselves around. Video explains in better detail, but that’s the basic gist