An interesting article on the history of the Assassins sect, who murdered their way through the Middle East from the late 11th to the 13th century, quotes my book Assassins' Deeds. A History of Assassination from Ancient Egypt to the Present Day (Reaktion).
A breakaway sect of a breakway Muslim sect, the Assassins killed many prominent Muslims, sometimes in cahoots with the Crusaders then trying to establish a Christian kingdom in the region. Even the great Saladin was afraid of them.
But they also murdered Crusaders such as Conrad of Montferrat, who had just been elected king of Jerusalem. The Assassins later apologised to Conrad's successor for their deed, and, to make up for it, offered to murder any enemy he chose to nominate. The order was eventually destroyed by the terrifying Mongol hordes led by the descendants of Genghis Khan.
Marco Polo, without any first-hand knowledge, told racy tales of how young men were recruited to the order in a valley like paradise inhabited by the world's 'most beautiful damsels' whose favours could be enjoyed by those prepared to commit murders when ordered. He also said they did their killings under the influence of hashish, a story which led to them being dubbed 'hashishin' which morphed into 'assassin'. There's not much sign that any of this was true, and, if anything, the Assassins' regime was probably rather puritanical.
You can find the article here: https://history.howstuffworks.com/world-history/order-of-assassins.htm