After all once the Dark Side has you it doesn't want to let go. Perhaps some sort of epic duel against a more powerful Dark Jedi. If any of the other characters are force-senesitive and don't yet use skills they should start against him. You could kind of do a dark side favor type of thing the opposite way. Such that they get extra to help him return then at the end all the additional powers are gone, but they know the possibility is there.
Evil is easy that's the point.