Well, it's worth noting that 2E's removal of them didn't stick. They were quickly shoehorned back in under the names of "tanar'ri" and "baatezu," transparent attempts to appease the people claiming D&D was Satanic*.
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Secondly, the entire concept to me risks deprotagonizing the game world and everything in it. If demons, why not angels? Why are demons priviledged to directly intervene and wage war if the forces of good are not?
Last time I ran an invasion it was from an alien LE plane (not Hell). I've always wanted to have a modron invasion but haven't gotten around to it. Yes, the party would be fighting being of pure LAW trying to turn the prime material plane into a world of clockwork lawfulness.Surely the forces of CG are no easier to control than the forces of CE? Why only invasions from the Abyss?
Why don't you encounter celestials as often as infernals?
Evil doesn't want to sweep mortals away. They want to corrupt them. Can't corrupt the non-existent.And if the servants of the gods, then why not the gods themselves? And if that, why are living humans particularly important, since the upper and lower planes could presumably sweep away mortal forces without much of a thought?
I think you're right. D&D needs loads of weird monsters. But no specific monster is essential. If you removed a lot of the iconic and mythological monsters it would feel significantly less like D&D. But the game can stand the loss of, say, the beholder and it wouldn't really matter.They are cool and a nice option, like undead, but certainly no group of baddies is essential for D&D to be D&D. Even dragons. Or orcs. Or goblins. Or kobolds.
Krynn has no orcs. I can't remember if it has demons or devils.