The Hashshashin

The Hashshashin were the first true assassin's guild. They were a highly selective and extremely secretive brotherhood. Led by an enigmatic and mysterious 'Old Man of the Mountain', these individuals systematically assassinated proponents of the Third Crusade on both the Crusader and Saracen sides.

They have been accused of being fanatics (much like Al Qaeda). Their leader, the ‘Old Man’ has also been accused of utilizing the drug hashish to manipulate his followers to his will.
First-hand historic accounts tell a different tale, one of honor and the ultimate pursuit of peace. Respect was given to them by both sides at the end of the conflict.
These individuals saw that there is no need for war—ever, and that the execution of a select few individuals could prevent or end wars.

They proved that much by ending the Third Crusade.

(I neither own the rights to Assassin's Creed nor have plan)

~Courtesy of the Nereverine