Assassination
The Steward's Fear Nightmare #3 (x1)
Objective
The Steward's Fear Nightmare
Plot.  

When Revealed: Attach Assassination to the hero with the highest threat cost. (Counts as a Plot attachment with the text: "Each enemy gets +1 Attached hero cannot be healed. Damage from undefended attacks and A Knife in the Back must be assigned to attached hero. If attached hero leaves play, the players lose the game.")