Go to the top level of the castle, manually attack the ghost, and it will trigger conversation on its turn. After that, do the same for the 2nd ghost encircling the castle perimeter. The last ghost is in the cellar. Make sure this is all at night, of course. Go to the moathouse back entrance, kill the snake there, and retrieve the body parts. Bring it back to the ghost at the top level, attack him to initiate conversation again, quest will be solved pretty quickly.
