During the Middle Ages, various branches and sects emerged within Islam. One of them was the Ismaili sect, known for its strong esoteric beliefs. Its followers believed that the rightful heir to the Prophet Muhammad was his cousin Ali, considered the first in a line of Imams. One of Ali’s successors was Ismail, the seventh Imam, who either died—or, according to some, went into hiding—in the 8th century.

Over time, the Ismaili sect endured persecution by other Muslim states and, by the late 11th century, split into two branches. One of these was led by the Persian Hasan-i Sabbah. In 1090, Sabbah, who became the Grand Master of this faction, seized the strategic fortress of Alamut in northern Persia, from which he began expanding his control over other castles. Shortly after, he sent his men to support Nizar, a claimant to the Egyptian caliphate. Although Nizar ultimately lost, Sabbah’s faction survived and became known as the "Nizari" sect.

The Nizaris organized themselves into hierarchical ranks under their Master and underwent rigorous training in espionage and infiltration. They became infamous for assassinating influential figures, including two Abbasid caliphs. To maximize terror, these killings were often carried out publicly, frequently in mosques, striking fear into both Muslim and Christian rulers—many of whom were forced to negotiate with them.

The calm with which Nizaris accepted death after completing their missions led contemporaries to believe they were drugged with hashish, earning them the name "Assassins"—interpreted by some as "consumers of hashish," though other sources claim the term means "followers of Hasan."

Sabbah died in 1124 and was succeeded by a line of Grand Masters, also known as Agha Khans. They continued to challenge powerful Muslim and Christian leaders, including Saladin, preserving the sect's elusiveness until 1256. That year, Mongol forces led by Hulagu Khan, grandson of Genghis Khan, captured the Assassins’ fortresses. Alamut itself was destroyed, and many Nizaris, including the family of Grand Master Khur-Shah, were killed.

Only one of Khur-Shah’s sons and a small number of Nizaris survived, keeping their identity hidden to ensure their survival. The title of Agha Khan was preserved through Khur-Shah’s lineage and is currently held by Prince Rahim al-Hussaini, known as Agha Khan V.