Feather

In Assassin's Creed the use of a Feather is a common way to prove that a target of an Assassin is dead. By rubbing a feather in the blood of the slain, Altair and other assassins show proof of their accomplishment.

In the Rich District of Damascus during the Abul Naqoud mission, one of the other assassins who asks Altaïr to join him in assassinating four of Abul Naquod's personal guards says, "I hear there is a feather on Abul Naquoud's head," implying that he is marked for assassination.