Here’s what Wikipedia says about the assassins:
In general, Ibn Sabbah [their head] was a master of falsification. Sometimes he used no less effective methods of persuasion or, as they now call, “brainwashing.” In one of the halls of the Alamute fortress, over a hidden hole in the stone floor, a large copper dish was installed with a carefully carved circle in the center. Upon the order of Ibn Sabbah, one of the Hashashins was hiding in the pit, sliding his head through the hole cut in the plate, so that on the side, thanks to skillful makeup, it seemed as if it had been cut off. Young adepts were invited to the hall and demonstrated to them the "cut head". Suddenly, Ibn Sabbah himself appeared from the darkness and began to perform magical gestures over his “cut head” and to pronounce mysterious spells in an “ununderstood, alien language”. After that, the “dead head” opened his eyes and began to speak. Ibn Sabbah and the rest of the attendees asked questions about paradise, to which the “cut head” gave more than optimistic answers. After the guests left the hall, the assistant of Ibn Sabbah was cut off and the next day she was displayed in front of the gate of Alamut.