I think that Evil Gods exist in DnD historically for two reasons. 1) To give us evil clerics 2) Because people were throwing all sorts of ideas at the wall to see what sticks. That's why we have dozens of overlapping deities, because people just made up a new force for their adventures constantly.
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However, in DnD, "gods" are the highest tier of beings and far more powerful than the demons and devils
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Which rounds us back to the point that has been brought up a few times. Other than "because the game was built with this fact" what value do we get from "God" being a more powerful and higher order of being?
I feel that you are straddling editions here.
In classic D&D (ie OD&D, early AD&D), and in 4e, there is no difference in power between gods and demon princes/archdevils. And both have clerics.
Lolth is a demon queen (see eg module D3, and the original FF) who is also a god of the Drow, with plenty of clerics. In DDG she is repurposed as a Lesser God, but as you noted upthread, and as I have noted upthread, in DDG
all the demon princes and archdevils were repurposes in this way. Here is the opening of the Lolth entry in DDG (revised version (ie no Elric or Cthulhu), p 92):
The dark elves worship demon lords from the Abyss. The best know example is the worship of the Demon Queen Lolth.
Module T1 The Village of Hommlet has as its final villain Lareth the Beautiful, a 5th level clric of "the Demoness Lolth". And (contra
@Helldritch) Lareth has a full suite of spells, not just 1st and 2nd level ones.
I don't have as much familiarity with a range of 2nd ed AD&D materials, but to me it doesn't seem to wildly depart from earlier AD&D: the module Dead Gods has a priest of Orcus, the half-ogre Quah-Nomag (who gets a full suite of spells up to 5th level). I'm not the biggest fan of Planescape, but it is evidence of what was accepted in 2nd ed AD&D.
Then in 4e, Demogorgon (a demon prince) is the same level as Torog (an evil god): both are 34th. Lolth and Vecna (two evils gods, in 4e) are both 35th. Imix, a Prince of Elemental Evil, is 32nd. There is no wild disparity of power here.
In the Forgotten Realms Bane is a mortal who ascended and became a greater god. In core 4e and 5e's Exandria he was a core and non-ascended god.
Asmodeus has been at various times an archdevil which gave him the powers of a lesser god when on his own plane (1e Manual of Planes), a greater god (2e Book of Hell), an angel who became a god (4e).
In DDG Asmodeus is said to be a Lesser God with no "on his own plane" limitation.
In core 4e, Bane is (perhaps) a mortal who ascended:
Bane was either a mortal hero or a demigod who slew Tuern, the previous (and less disciplined) god of war . . . (The Plane Above, p 51)
I know that Dragon 372 says something a bit different, but that is consistent with the general approach in 4e of presenting its mythology in multiple overlapping but non-definitive versions.
Fiends don't get clerics in base lore.... ever.
As has already been posted, this isn't accurate. In AD&D there are clerics of fiends, as I've shown above. In 4e there are clerics of devils but not of demons, reflecting the particular cosmology of that setting. There is also at least one "paladin" of Asmodeus, namely, the Duergar Murkelmor (in module H2 Thunderspire Labyrinth).
But Yeenoghu, Demogorgon, Orcus et al all have cultist followers who are functionally clerics and serve the same role in the fiction as the Gygax-era anti-clerics and evil high priests (eg the Deathpriest of Orcus in the MM; the Berserker Prelate of Demogorgon in the MM2; Zaiden, Yeenoghu's cult leader, in Dragon 364; in H2, Maldrick Scarmaker is statted as a warlock, although there is no provision for a PC warlock to be a warlock of a demon lord).
If written today many of the clerics of fiends would be warlocks. Clerics of fiends and celestials more or less disappeared within the reinvention of warlocks
I don't agree, based on the examples I've given in the previous paragraph.
Yes they had these clerics even in 1ed. But remember that these clerics only had access to 1st and 2nd level spells top. It was only through the introduction of Banak that they finally got "full power " clerics. I bet that if warlock existed then, that these clerics would not even have seen the day...
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And no they did not have clerics but they did have cultists. And the few true clerics they had, were stuck with 1st and 2nd level spells...
This isn't accurate, as per the earlier parts of this post.
No, they were never intended to be gods in their own rights. That is why they fight the gods because they want the power that the gods have. They will tempt, trick and fool mortals into believing that they are as strong as the gods, but they are not. But they certainly want you to believe it. That is canon.
This isn't accurate either. In AD&D, as soon as the concept of a "god" came to matter (ie in DDG), the archdevils and demon princes were declared to be gods.
An evil paladin used to be an Antipaladin. He was a paladin corrupted by evil, usually demogorgon as only him had the full power of the Abyss as the "Prince" of all demons... Yes, canon there are antipaladins of other demons such as Orcus.
There are no canonical anti-paladins in 1st ed AD&D (unless you count Death Knights, whom the Fiend Folio (p 23) conjectures are created by Demogorgon), and I don't know of them in 2nd ed AD&D either. 4e D&D has a discussion of paladins of evil gods in its DMG (p 163).
There is a fan-authored anti-paladin in Dragon 39, but it is not especially connected to Demogorgon.
You can run a true neutral cleric of Nerull.
While I can see your rules argument in 3E terms, I agree with
@Maxperson and
@Chaosmancer that this is not really consistent with the established fiction for Nerull.
if we look to the time period when many of these Demon Lords and Archdevils were created, them being equivalent for the most part to the gods... is exactly what was intended. Sure, they didn't have cavaliers and paladins, but they did have clerics and secret temples. So, we really are losing nothing by going back towards that model that I can find.
Agreed.