Ferondo, having swallowed a certain powder, is entombed for dead and being taken forth of the sepulchre by the abbot, who enjoyeth his wife the while, is put in prison and given to believe that he is in purgatory; after which, being raised up again, he reareth for his own a child begotten of the abbot on his wifeThe sleeping powder in it is described of having been obtained from the Old Man of the Mountain. The Old Man of the Mountain was the leader of the murderous sect of Assassins (in Syria and Persia as I recall). It is another example of the vast frame of reference a reader was expected to have.
From what I can gather, idea of the Old Man of the Mountain comes from the Travels of Marco Polo, which was published in the early part of fourteenth century. (Bocaccio was writing around 1350.) Polo recounts a story he heard that the Old Man would drug his young followers with hashish, lead them to a "paradise", and then claim that only he had the means to allow for their return. This is what imbued the Assassins with their legendarily self-sacrificing loyalty.
Two things are pretty funny here. First that the character in the story is knocked out with powdered cannabis. Second that the Old Man of the Mountain used it to convince people they were in paradise, but the Abbot uses it to persuade someone he is in Purgatory. What could be more Catholic than that?