"The Assassins were a radical group of Shi'ite Muslims who practiced terrorism against their enemies. If a leader, usually Muslim, opposed them, they murdered him. In 1173, the Old Man of the Mountain, leader of the Assassins, sent a peace envoy to Amalric, the Crusader King of Jerusalem, proposing an alliance. On his way back from the conference with the King, the Assassin envoy was attacked and killed by a band of Templars who were led by a one-eyed knight named Walter de Mesnil. What was the background of this event?