Hassan i-Sabbah was a 11th century Ismā'īlī Nizārī missionary. The Ismā'īlī are a branch of Shi'a Islam, the Seveners are a branch within the Ismailis, and the Nizāriyya are a sect of the Seveners. They broke off because they believed that the successor Imam to the Fatimad caliph al-Mustansir should have been Mowlana Nizar instead of his brother Al-Musta'li. Hassan i-Sabbah was the leader of the most notorious group of Nizari, the Hashshashin. The Hashashin were generally known for terrorising the Abbasid caliphs, among others, through targeted political killings. The English word "assassin" comes from the sect.
Hassan founded the group in 1090 in the mountains at Alamut, south of the Caspian Sea. He is best known to the West through the writings of Marco Polo in 1279. At this time, Hassan was remembered by the name "Old Man of the Mountains". Marco Polo's story has become famous:
"Recruits were promised Paradise in return for dying in action. They were drugged, often with materials such as hashish (some suggest opium and wine as well) then spirited away to a garden stocked with attractive and compliant women and fountains of wine. At this time, they were awakened and it was explained to them that such was their reward for the deed, convincing them that their leader, Hassan-i-Sabah, could open the gates to Paradise."
The story is likely more myth than reality, but what is most interesting about the group is how feared they were, given their small numbers and limited geographic range. They were eventually all but wiped out by the Mongols, but for a secret society, their power at its height was equal to that of the caliphs. They were a very tightly knit, semi-mystical group whose members gave their utter devotion of Hassan, generally killing the person who they were assigned to kill, and then waiting to be killed themselves.
Also likely just a myth are the reported final words of Hassan i-Sabbah:
"Nothing is true. Everything is permitted."