He's probably dead or in hiding at a location so far away we haven't heard of it.

Who he is and what he tried to do will be known across the continent so I have no doubt that there would have an enormous reward placed on his head so any faction with capable fighters and trackers would have been after him. He's not like Arkhan where he would be able to teleport out, at least from what he know he doesn't have spells that would allow him to escape from someone's clutches the moment he's found so eventually he would have found himself in a fight he can't win or run from.

This is of course up to Matt but I don't really feel like Silas should show up again.