You can enter Trighton via several portals located in : Dungeon of Doors, Elder Halls of the Beholders, Isle of Ice Crystal Forest, Isle of Death Tomb, and finally Isle of Secrets. The locations of those portals are close to the "boss creatures", carrying a component for the Sendauntra portal opening ritual. As for the Trighton quests, it is the final of the "A Merchants Tale" line. You finish it by killing the Rhule Tower Guard Boss. *AB is approximate.