Complex sentence: After a time, the assassin Artemis Entreri tracked Regis down and during their treck through the, then duergar controlled, Mithril Hall stole him back to Calimport.
Split into simpler sentences: After a time, the assassin Artemis Entreri tracked Regis down. During the treck through the, then duergar controlled, Mithril Hall Artemis stole him back to Calimport.