Nonetheless, to start his questline you must reach the Altus Plateau. It's important to do this via the Grand Lift of Dectus, which you can do by collecting both halves of the Dectus Medallion.

This Black Knife Assassin is located inside the Sage's Cave. This cave is located in Altus Plateau, in the body of water inside this zone. If you travel north from the Altus Plateau Site of Grace ...