It actually took me awhile to figure this out when I was new to the game.
It is an Assasin Favor item. You need to click on the tab to go from basic favor items to General/Assasin Favor items.
The same thing works for general (as in assassin/general) items. If you are a general, you have to click on the 2nd tab when you select a favor item to get general only favor.