Magician: "I believe it is customary to announce your presence when entering the home of another."
Ghost: "This home does not belong to you."
While this was strictly true, the Magician preferred to operate under older laws. Why should an vacant property not be taken up by someone who needed a home? Letting it sit, benefiting only a bank, was surely the greater crime. He opened his mouth to defend himself, but the ghost spoke first.
Ghost: "You seem to have a habit of taking things which are not yours for the taking."
The Magician kept his face carefully neutral as he answered, not letting the spirit see his anger at being interrupted. It was best to find out what the ghost knew before feeding him any more information to make guesses from.
Magician: "I seem to be at a disadvantage. I would know who I speak with, surely I have that right."
Ghost: "My true name is ancient, lost to time. Today, I am referred to by many names across many cultures."
He paused there and studied the Magician's face, as if peering through to his soul.
Ghost: "You, Septimus Iuventius, knew me as Orcus."
The Magician worked carefully to keep his face expressionless. So the spirit knew his name, and where he came from. What else did he know? In contrast to his internal worry, he twisted his face into a derisive smirk.
Magician: "You take the name of a god?"
Orcus: "I am a god."
Magician: "There is no such thing. There are mortals, and those who transcend mortality, but no gods. But you didn't come here to debate theology."
Orcus: "No. I came to issue an ultimatum. Return what you have taken, or the retribution will be swift and without mercy."
Orcus: "Find it. Or, perhaps, you do not take me seriously."
As Orcus spoke, the words seemed to fill the Magician's mind, evoking images from memories that were not his own. Some ancient magic was at work here, and it scared him; even if he had thought it wise to speak, he didn't believe that the magic would let him.
"Listen, for I would have you understand your place. Long ago, those who came before walked this soil. There was conflict, and war. The peaceful Alakar stood above, apart, protected by their superior intellect and technology. But those below grew too many, and even the great citadels of the Alakar began to fall. So they took to the stars, leaving behind the world of chaos on which they had grown."
~ * ~
And the plot thickens!
For your convenience, here's the wiki entry for Orcus. He was a Roman god associated with the underworld, and known for exacting punishment for broken oaths. Seemed appropriate to me.