You seem to be into EU stuff, so I am assuming you want it to fit into continuity to some degree. In that case, who trained this young twenty something year old Jedi who is turning bad? Why is he turning to the darkside, what is influencing him to do so?
From what I have read of the EU, the galaxy is sorely lacking of Sith/Dark Jedi after the death of Vader and Palps, until Luke's Jedi Academy trains enough new Jedi that some start to fall.