al-Hassan ibn-al-Sabbah, founder of the Ismailite Assassins
Hasan-e Sabbah (there are many spellings of his name) was the leader of an Islamic sect known as the Nizari Ismailiyah or Ismailites, in the 11th Century. He was a devout man, having studied theology in Rayy in Iran, and became a disciple of the Ismailite faith around the age of 17.
It is known that he went to Egypt in 1076 for a time, to continue his studies, before reurning home to boost Ismailite interests and numbers. In 1090 he and his followers seized the fortress at Alamut in the Elburz Mountains, and set up garrisons in impregnable mountain strongholds, which became known as the 'Castles of Assassins'. Following this, he proceeded to take over large amounts of territory in what is now Iran and Iraq, and it was said that his very name could strike fear into the hearts of the Caliphs.
He was responsible for creating the cult status of the devoted Ismailite shock troops known as the hashshashin (which entered the English language as "assassins"). It is likely that the word is drawn from his name (as opposed to the commonly-held view that he heightened their religious fervour with hashish). As a devout Muslim, he (along with others of the group) were opposed to the use of narcotics, and associations of hashish are drawn from the uncertain testimony of Marco Polo, and not likely to be historical fact.
As the leader of this widely-flung state, he settled down to a peaceful reign following the last seige at Alamut in 1118. As something of an aesthete, his regime was puritanical (even having his sons executed following their transgressions).
He died in 1124 at Daylam.
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Encyclopædia Britannica