Further on the
cleric issue: in OD&D v1 Men & Magic, we are told that
Clerics of 7th level and greater are either "Law" or "Chaos", and there is a sharp distinction between them.
The list of
Chaotic beings a couple of pages later includes
Evil High Priests, but nothing is said about where their power comes from. I don't think there are any demons or devils presented in those original three books.
@Doug McCrae probably knows more about this?
OD&D
I think this excerpt from Jon Peterson,
Playing at the World (2012) is correct:
Dungeons & Dragons has almost nothing to say on the subject of the divine, on the god or gods that might be revered by Clerics; although later supplements fill this gap, the 1974 woodgrain box gives us priests entirely without religion.
The balrog is the most demon-like creature in 1974 D&D, in my view. Demons and demon princes – Demogorgon and Orcus – first appear in
D&D Book VI Eldritch Wizardry (1976). Gods are in
Book VII Gods, Demi-Gods & Heroes (1976).
The 1974 D&D cleric is heavily influenced by Christianity, with spells such as Turn Sticks to Snakes. It seems plausible therefore that the "Anti-Cleric" (Book I, pg 34) who casts reversed versions of some cleric spells is a Satanist. Another, similar, possibility is suggested by the entry for the Aztec god, Mictlantecuhtli, in Book VII: "Any person that worships this god is definitely worshipping Chaos itself, since Mictlantecuhtli craves death."
Fiction That Inspired OD&D
Poul Anderson's
Three Hearts and Three Lions (1953 novella, 1961 novel) and Michael Moorcock's 60s and 70s Eternal Champion stories are the two most important sources for alignment in OD&D.
In Anderson's novel most of humanity, Christianity, Judaism, and Islam are on the side of Law. Witches, warlocks, demons, and the god Pan are on the side of Chaos. It's hinted, but not made explicit, that Satan is the ultimate leader of the forces of Chaos.
'In olden time,' said Hugi, 'richt after the Fall, nigh everything were Chaos, see ye. But step by step 'tis been driven back. The longest step was when the Saviour lived on earth, for then naught o' darkness could stand and great Pan himself died.
Christianity, Judaism, even Mohammedanism frowned on witchcraft, that was more allied to Chaos than to orderly physical nature.
He [Alfric, leader of the Faeries] was not the head of the enemy. Morgan le Fay outranked him, and beyond her must be others, clear on to a final One whom Holger did not wish to think about.
This passage from Moorcock's Elric novella "Sad Giant's Shield" (1964) describing "the camp of Chaos" is strongly reminiscent of the Christian hell and its demons:
The towering Ships of Hell dominated the place… Shooting flames of all colours seemed to flicker everywhere over the camp, fiends of all kinds mingled with the men, the evilly beautiful Dukes of Hell [Lords of Chaos] conferred with the gaunt faced kings who had allied themselves to Jagreen Lern and perhaps now regretted it… The noise was dreadful, blending of human voices and roaring Chaos sounds, devil's wailing laughter and, quite often, the tortured scream of a human soul who had perhaps relented his choice of loyalty and now suffered madness. The stench was disgusting, of corruption, of blood, and of evil.
Modules
The clerics in
B2 The Keep on the Borderlands (1979) worship demons. There is a "demon idol" in the "chambers of the evil priest." Anyone who picks up the "relics of evil" in the "chapel of evil and chaos" falls under "the influence of a demonic spell." One of the clerics plans to sacrifice the captive medusa as part of "a special rite to a demon."
In I
2 Tomb of the Lizard King (1982), the clerics allied with the lizard king Sakatha are "demon-worshiping." The Type I demon, Grzzlat, "was ordered by a cleric to carry a few spells up to this plane." I think that's a reference to this passage in
AD&D 1e Deities & Demigods (1980): "Third, fourth and fifth level spells are granted by the supernatural servants or minions of the cleric's deity."
EDIT: Corrected letter code for module I2.