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One of the things I find interesting is how devils and demons are portrayed in Advanced AD&D 1e. Devils and demons are, for the most part, humanoid creatures of great malice and evil who are distinguished primarily by alignment (Lawful Evil versus Chaotic Evil) and location (Hell versus the Abyss). There are lots of each of them and they are surprisingly similar.
In the primary sources of the Judeo-Christian tradition, the Bible, we get a different picture. There is surprisingly little information about demons or devils, except that they exist. If we only look at the Christian New Testament, we can see that the existence of both devils and demons is assumed so there is little offered in the way of proof or description. If we leave out the book of Revelation for a moment, we discover the following:
Devils: There is actually only one devil mentioned and that would "Satan" (insert Church Lady voice). There is no physical description given of the devil nor are we told where he lives. The devil is described as the adversary, the wicked one, the Accuser, and the Father of Lies. He is also called the prince of this world and the prince of the power of the air. He is said to have been cast out of heaven and Christian tradition has held that the devil (i.e. Satan) is a fallen angel, maybe even the chief angel. The devil is said to disguise himself as an angel of light in order to lead the faithful astray. While possession tends to be the purview of the demons, the devil is said to have entered Judas to lead or tempt him to betray Jesus. There is not enough detail to conclude that Judas was actually possessed, many have drawn that conclusion.
Demons: There are many demons in the Christian New Testament. There are several cases of demon possession, with Jesus driving out the demons. Demons are not described but based on the accounts, they seem to be evil spiritual creatures that are capable of possessing or taking over humans and other animals (a herd of swine is the given example). It is clear that there is a connection between demons and the devil, but there is not much information given.
Beelzebub: There is a mysterious reference in all three Synoptic Gospels (Mathew, Mark and Luke) to a "Beelzebub." He is referred to as the Master of demons, but there is also a hint that Beelzebub is either Satan or closely allied with him. However, these passages all describe how Jesus is accused of using the power of Beelzebub to drive out demons. The point of these passages is not to describe the relationship between Beelzebub and Satan. Unfortunately, Queen had it wrong, Beelzebub has a demon set aside for Freddy Mercury, not a devil.
Whereas the devil seems to live somewhere else besides earth, only to come here to tempt us, demons are presented as being all around us, lying in wait to possess us and drive us to madness. One has the power of temptation and deception while the other uses brute force to possess us. Neither is really much like the AD&D 1e Monster Manual, not that it should be.
All this is stuff that I knew from years of theological study, including a lot of reading in the original Greek. What I had forgotten is how little information is actually in the Christian scriptures about Satan (the devil) and demons. Most of what the average Christian believes about devils, demons and even Hell comes from extra-Biblical sources. There is more about these matters in the book of Revelation, but even the book of Revelation provides scant and unclear details about such things. But that is a story for another post.