The mortal enemy to the assassin order, a Haystack out of which one cannot get out. The one weakness they never saw coming, would be their doom.
To exploit this weakness in the game, complete the quest "Childhood Sweetheart" and then fast travel to Colchestre. There, from atop the spawn location, simply jump into the Haystack. You shall see, that is the last thing you'll do. I sure hope the TemplarOrder does not get a hold of this..