I read The New Rebellion a Star Wars novel set 13 years after Return of the Jedi. Luke's big bad apprentice is chilling on a planet making droids. Does he ever do anything in future books?
After the events of The New Rebellion, Brakiss was forced to flee, ending up in the Deep Core working as a go-between for some Imperial Remnant warlords. He eventually established a training facility, the Shadow Academy to train Dark Jedi with the intent of assaulting Luke Skywalker's training facility on Yavin 4, as shown in the Young Jedi Knights series of books. His forces were defeated and he died when an internal conflict within his forces resulted in the Shadow Academy's self-destruct mechanism being engaged.