A Skull inhabited with the spirit of Mithra.
Cavelli, still apparently oblivious to the voice, begins to muse. “I’ve heard the skull you seek can speak to the minds of certain adepts, but Mithra cannot always be trusted. - Backstory codex - Hunter (RPA-BC)
The Ossuary is where you run into Cavelli again looking for the skull of Mithra. A voice in your head tells you to lose Cavelli.
In Kulbak Prison the Skull speaks to you again, telling you to find a map inside a Construct.
You find the Gravestone with the strange markings you remember that your father taught you. Inside you find a solid green orb. Cavelli shows up with several other figures from the Starlit door