(Cassiel, the ruling spirit of Saturday, bears a distinctly demonic aspect in Barrett's Magus)

Cassiel is not strictly speaking a demon of hell, but a ruling spirit of Saturday. When works of magic are conducted that fall within his authority, they are done on his day of the week, and his power is invoked to insure their fulfillment. There is often a fine line in Western magic between a spirit and a demon, as the illustration of Cassiel from Francis Barrett's The Magus indicates. Physically, nothing distinguishes Cassiel from dozens of infernal demons described in the Goetia. He is shown as a bearded king with wings and scaly skin, who rides on the back of a winged dragon, and carries in his right hand an arrow. Cassiel is described in the Fourth Book of Occult Philosophy (which is a grimoire of magic falsely attributed to Cornelius Agrippa) as "A King having a beard, riding on a Dragon."

Francis Barrett stated concerning the spirits of Saturday "the nature of them is to sow discords, hatred, evil thoughts and cogitations, to give leave to kill and murder, and to lame or maim every member." This is demonic enough that Cassiel can safely be classed as a demon for all practical purposes. Barrett adds an interesting footnote about the kingly appearance of Cassiel: "Those spirits who appear in a kingly form, have a much higher dignity than them who take an inferior shape; and those who appear in a human shape, exceed in authority and power them that come as animals; and again, these latter surpass in dignity them who appear as trees or instruments, and the like: so that you are to judge of the power, government, and authority of spirits by their assuming a more noble and dignified apparition."