"The castle has one ghost." - Acceptable?
What about "a phantom" since it is so close to the Greek and is a synonym for ghost? The "Phantom of the Opera".
Just my opinion but I think "phantom" would be more widely used ing a literary sense or as an adjective. "Phantom limbs" comes to mind. Hope some else can come in with a better explanation.
You are probably right, I just liked its similarity with the Greek word.