Yes, the Mask of Deception drops from the Ghoul Assassin in LGuk. I guess it isn't important for the person who made this post, but for future people it is helpful I guess.
You go to LGuk Dead side. Find a map at eqatlas.com. There is a marking on it called "*** sup." The Ghoul Supplier is the Assassin's placeholder. *** sup refers to the camp. It isn't a hard camp, but no 30- can handle it solo. I just sneak/hide'd to the area around the room (mobs at camp see invis) and waited for someone to come by. Then I just asked them to drop the Assassin for me and voila.
However, I noticed upon looting it that I don't have to have it equipped to click it for the proc. Why is that? Can others just not click?