Worthy of note, "hashishiyyin" is a sobriquet along the lines of "dirty hippy" or "filthy stoner" as the Fedeyeen were legit not-fucking-around terrorists who lasted as long as they did and shaped Islamic society as much as they did through seriously ruthless, underhanded asymmetrical warfare.
Hashish or Hashisha is the Arabic word for hemp, which is latinized cannabis sativa. Its variety is Indian hemp or Cannabis Indica, have been known and used in the Near East since ancient times as a drug with intoxicating effects. The earliest express mention of the word hashish contained in "at-Tadhkirah fi'l Khilaf" by Abu Ishaq ash-Shirazi (d. 476/1083). The use of hashish grew in Syria, Egypt and other Muslim countries during 12th and 13th centuries among the inferior strata of society. Numerous tracts were compiled by Muslim authors, describing that the use of hashish would effect on the users' morality and religion. Consequently, the users of hashishqualified for a inferior social and moral status, similarly to that of a mulhida, or heretic in religion. Neither the Ismailis of Syria nor the contemporary non-Ismaili Muslim texts, which were rigorous towards the Ismailis, ever attested to the use of hashish among the Nizari Ismailis.
Most any "secret sect of killer monks deep in the mountains" paradigm you've ever read or watched is the echoes of the fear of the Nizari fedayeen ("those who sacrifice themselves"). They were pretty much the world's premier terrorist organization for about a hundred years.