Training

In real life, Hassan-i Sabbah was, of course, not a master of martial art, but a scholar and brilliant teacher.

He taught his students how to kill his adversaries. (That's the origin of the English word assassin!)

And his method was remarkable. The traditional account is like this: Potential students were sent to Alamut and were brought up in a paradise (in fact, a fantastic garden with wine and beautiful girls and what not). In there, they were kept drugged, and Hassan occupied this garden as a divine emissary. As the old English idiom says, when the time is ripe, the drug was withdrawn, and they were sent out from the gardens and flung into a dungeon. There they were told that, if they wished to return to the paradise they had so recently enjoyed, it would be at Hassan's discretion, and that they must therefore follow his directions exactly, up to and including whipping off whoever they were told, and sacrificing their life if needed.

It seems a common trick of religious leaders.

PS. The story of Hassan and his group sounds great, but I'm not sure how reliable that is as compared to the fictional account by Jin Yong. Most of what we know derives from Marco Polo, who visited Alamut just after it fell to the Mongols in the thirteenth century.

And, you know what, after reincarnation, Marco Polo became Baron Munchausen.