Demons part 3- The Lords of Nightmare and a Belated Overview of AD&D Demons as a whole.

OG Demon Lords
AD&D Demon/Devil Lords are among, if not the, most powerful beings in the monster manual. As such they rarely get used directly in most games, and mostly exist as Official D&D™ lore dumps you can look up online yourself far better than I'd explain 'em.

OG Demons- I find it interesting that the AD&D demons pretty much all have an amount of weight they can move via telekinesis listed in terms of gold pieces- like, that is prominently mentioned for demons. Sure they also tend to have levitation, and some sort of fear effect, often darkness, pyrotechnics, and polymorph self, and a variety of utility effects like language reading, detect magics, etc, but it really seemed like the big question about demons was 'how much treasure can it haul.'

Unlike many monsters, have motives ascribed to them.
Type I demons  like human flesh and precious stones and metals and are described as 'stupid and prone to listen to bribes.'
Type II demons are like type I, and animostity between these two types is noted.
Type III- n/a
Type IV- Noted to have names and to be 90% likely to listen to offers of great rewards for small services, as well as a fondness for human flesh and blood.
Type V- Noted to be feared by lesser demons as they are domineering and cruel, having names like type IV demons, and desiring the sacrifice of strong warriors to them.
Type VI- "With proper invoking, offerings, and promises, type VI demons might be convinced to co-operate with a character or a group for a time. Naturally, the demon will attempt to assume/usurp command at every opportunity."
Succubi- Rule lower demons through wit and threat.

By contrast Devils seem less inclined to negotiate as they are locked into a  rigid command structure of rulership.
Barbed devils are alert guards who immediately cast unauthorized creatures into cells for torment, Bone devils delight in making less powerful creatures suffer, Erinyes are pursuers and abductors of evil souls and engage in bargaining and temptation of others, Horn Devils hate things stronger than themselves.

Demons may be summoned by any alignment and types i-IV cannot cross thaumaturgic circles, and will never willingly serve (meaning they only do tasks that they believe they are getting the best deal from) and are likely to carry off 'Liked' individuals to be favored slaves. They are also warded off by holy relics and symbols.

The point of mentioning all this is that they seem custom built to  be puzzle encounters, and potential ways to carry more GP in a dungeon. Forget having slugathons with demons over moral differences, the way to deal with AD&D demons is to try to find their name, hide in/trick demons into thaumaturgic circles, ward them off with holy items, and bribe them. This is basically what I was doing with Ifrits and Djinn already, so extending it to demons is my new go to.

3.5 Demons
 By contrast, the 3.5 entries merely  talk about how the demons prefer to battle, the Balor entry even giving round-by-round tactics to employ. There is no mention of thaumaturgic circles, repelling demons with holy icons, and certainly no assumption that a player might ever interact with one in any way besides punching it. While it would be easy to point at this as yet more proof that 3.5/PF are games about combat math above all else, it's worthwhile to remember the whole 'satanic panic' thing made the executives of D&D explicitly adopt some guidelines to affirm a sort of 'moral center' to defuse the shrieks of the religious authoritarians. While they were not adhered to with unwavering obedience, I expect in addition to renaming demons and devils 'Tan'ari and Baatezu,' making interactions with demons solely a matter of 'good guy vs bad guy' may have played a part in the dumbing down of demons from puzzle encounters to combat encounters.

I'd be curious to see if the 5e statblock has any non-combat interactions laid out for demons.

Sunset Realm Demons
 Any immortal spirit being with 6+ HD and a dead or forsaken creator deity is probably called a 'Demon.' Things spawned from mortal nightmare realms are typically called Nightmares instead, but it's basically the same thing- a being that lost its original purpose and has grown corrupt, predatory, and self serving (or started out that way, depending on their origin.). There's no great point in categorizing all of them, and most spells of level 4+ probably have a type of demon competing with spellwisps to be unleashed onto the waking world. Binding a demon as a spell is probably the most common form of a demon pact, though if their names are found they could be summoned for all manner of things. While not inherently 'evil' as standard D&D assumes, many of their desires may be at odds with human experience (Two demons in my current campaign want, respectively, rare and impressive legs, and to encase living beings in prisons of unusual solidified objects. They want these things in the same way humans want to eat tasty food and to have satisfying social connections.)

Sunset Realm Demon Lords
There are two meanings for the term 'Demon Lord.' One is just words, applied to a demon who lords over things, or someone who lords over demons. These self-proclaimed or earned titles are basically just nicknames, no different from calling a king a 'Human Lord.'

The other meaning is in reference to things crawled from the deepest pits of nightmare, bad dreams with no dreamers, sprung from the edge of the unknowable darkness at the bottom of the collective consciousness that is the dream realm, and Demon God is more commonly used by sages who are wishing to be very clear on the matter (though it just obfuscates the issue in another way. Ah, language...). They are shaped by the dreams of all beings, and perhaps shape them in return, until it is unclear which created which, but unlike the desires of lesser demons, the Lords of Nightmare embody widespread feelings familiar to most people, and as such exist on a more transcendental scale compared to a demon that is basically just a rude spirit monster. Below are two such beings.

Unfortunately Mohawk Babyface was hated to death moments after the touching attempt to offer Isfrix a flower. A common demonic form of Cause Wounds/Magic Missile/Disintegrate/etc is stating things you hate about the target, invoking Isfrix, and then watching the target get scoured from existence. Isfrix hates everything so there's never restrictions on targeting, and most gods hate something, so they can't really complain about the occasional invocation of Isfrix from their clerics.

Isfrix, Lord of Hate- Possessed of equal and infinite hatred for all things, including itself, including the hatred for itself, including the hatred for the hatred for itself, etc etc, Isfrix is a mad and broken entity that seeks the destruction of all things, including itself. However, the hatred humanity(and other beings) engages in keeps the god alive and caught in a perpetual suicide, mashing its broken face against a mountain atop which the Sphere of Annihilation rests, regenerating faster than it can die and bleeding hatred into the world, which hate-stricken people refine into curses of all varieties through their own malign will. Hate will not vanish from the world until Isfrix is dead, but Isfrix cannot die until hatred has vanished from the hearts of all beings.

The race of beings Isfrix created are the Conqueror Worms, brightly colored little worms that have the ambitions, emotions, and intelligence equal to humans, but are trapped in pathetic and useless bodies that cannot sate their dreams... but can steal the bodies of others via sitting in the brains. They hate their creator, and are jealous of beings, but no simple villains are they- they are split into factions, those who wish to simply watch over the grave of Isfrix until the end of the world, those who wish to take what was denied to them from the other beings of the world, and those who wish to live in cooperation and transformation and break the cruel narrative Isfrix wrote for them back when the god was more sane and more actively malevolent.

I only realized I had just reinvented the plight of the Yeerks a few months later
don't sue me Applegate

Janus, Lord of Greed- The two-faced chthonic god of blood and gold, and the interplay between the two. Sacrifice blood to get gold, or gold to save your own blood- it's just like the interplay of violence and wealth that occurs even without the influence of demon gods. For a long time, Janus was accepted by many in the city of Oroboro, and still has influence on the folk of the deep places of the earth who tend to be rich on gold and violent monsters, and undead who pay for superior states of cursed unlife.

The Hell of Janus, a place where the servants wear chains of gold, the mountains are made of coins, and every last inch is stained with blood. The blackened towers of scabby gold have no connections to each other and simply compete to rise higher and higher. The promise of one day being lord of their own decadent tower keeps the lackey souls laboring eternally.

The flames of greed cannot be extinguished by throwing any amount of blood and gold at them, and so Janus, like Isfrix, will likely persist as long as a price can be set on life. Even the act of desecrating tombs of Janus worshippers spreads greed, for who would turn up their nose at the bounties of demon-gold beaten into grave goods? Areas that use a gold standard of currency are usually under the sway of Janus, and it is hard to say whether all gold came from Janus, or if Janus came from the bloody wars for gold.