Devils occupy the lawful end of the fiend spectrum: demons occupy the chaotic end. Unlike devils, which rarely stray onto the prime material plane except on a mission of malice, demons like to exploit holes in the cosmic fabric, popping through to ruin things for everyone on the other side. Thus, an adventuring party is much more likely to stumble upon a random demon than a random devil. There’s also the possibility that a demon has been summoned as a servant or ally by an evil spellcaster but broken free of its bonds.
Advanced Dungeons and Dragons numbered demons by type, but by the second edition, descriptive names were already supplanting numbered types (2E didn’t even use the word “demon,” probably cowed by rampaging fundamentalists). Fifth-edition D&D brings back the numbered types, but they take a backseat; each variety of demon is referred to primarily by name.
One trait that all demons have in common—which is described in the Monster Manual flavor text, not in their stat blocks—is that they can’t be permanently killed on any plane except the Abyss. If the player characters destroy a demon on their own home plane, that demon isn’t killed, merely dispelled, and it immediately re-forms in the Abyss in a nasty mood and with a new grudge. (more…)