Yeah, I think it's probably the most chilling of all the episodes - Peter Bowles is great as Balor and the concept of an immortal psychopathic sadist is excellent. Also Balor's fate is pretty chilling - to be locked away for all eternity, alone (the novelisation expands on the idea, in that the Alphans note that the painiting - which we later find out are depictions of the crimes - are not created in the asteroid. "No pigments, no paints, in fact, nothing at all" which leads them to wonder what sort of people could inflict such a punishment. Of course we later find out that the Progons couldn;t do anything else, as the immortality was irreversible). The end sequence indicates that the prison asteroid recaptures him, because it - like Balor is effectively immortal as it can regenerate.