Demogorgon is the big bad of big bads. As the self-styled Prince of Demons, he’s been making life difficult for players for more then thirty years. From what I can tell, he is one of the three most powerful monsters in all of 5E. I have a poster of Demogorgon, and I don’t even like demon lords. That’s how influential he is.
To Demogorgon, there’s no power like overpower. He rolls in like a force of nature, destroying anything he can see before his enemies even know he’s aware of their existence. He hates everything, including and especially himself, and his perfect world is no world at all. His followers generally worship him out of fear or insanity; either they don’t have sufficient control of their faculties to resist his commands, or they’re certain he’ll eventually triumph and they think it’s better to be on his good side before their inevitable destruction.
Typical Followers: Most people don’t want to destroy the world, so many of Demogorgon’s followers are people who went insane when they laid eyes on his symbol. They can come from all walks of life and the only creatures immune to it are creatures immune to insanity. That means it’s hard to say which people are more likely to follow Demogorgon than others, because there’s really no rhyme or reason to it. His intentional worshipers tend to be demons who think it’s better to be on the winning side even if it means the Abyss eventually disappears. They’re good antagonists at all levels, but not especially interesting, and the only monsters who follow him religiously (so to speak) are merrow, aquatic ogres unsuitable for most situations.
Atypical Followers: Properly, any non-demon who intentionally follows Demogorgon counts as “atypical”. He’d find his most ardent allies in creatures already inclined to destruction, like magmins or efreeti. Certain types of undead, wights especially, agree with his intent to destroy all life. Lamias might also indirectly make sense, relishing the sight of a prideful person brought low and working toward a different, subtler sort of pain.
Plots: Demogorgon’s most obvious plots revolve around his symbol, a shape capable of twisting the minds of any who so much as lay eyes on it. The symbol can find its way to anywhere, sketched by a cultist on the wall of a magical university, a court of law, or the ground in an alley on the edge of a bustling marketplace. A sudden influx of serial killers is cause for concern anywhere, perhaps surpassed only by a singularly powerful entity who falls under his sway. Remember, the symbol affects everybody—a dryad, mind flayer, or even angel can succumb to it just as easily as an elf.
From the Vault: There are a few destruction-based concepts in older versions, but the easiest to generally apply may be the entropomancer from 3.5E’s Complete Divine. It worked with entropy, here intended as collapse rather than chaos, and its trademark ability was conjuring and controlling an itty-bitty sphere of annihilation. The entropomancer is a prestige class intended for clerics or druids with training in Arcana, but in 5E we can create some new attacks and traits for monsters channeling Demogorgon’s powers of destruction:
Shard of Entropy. The creature targets one creature it can see within 30 feet. The target must succeed on a DC 12 Constitution saving throw, taking 5 (1d10) force damage on a failed save, or half as much damage on a successful one.
Entropic Field. Whenever a creature within 5 feet of this creature regains hit points, it treats any dice rolled to determine the hit points it regains as having rolled their minimum value.
Disruptive Field. Whenever a creature within 5 feet of this creatures makes a saving throw, this creature can use its reaction to force that creature to make the saving throw with disadvantage.
Annihilating Field. Whenever a creature within 5 feet of this creature takes damage, that damage increases by this creature’s Charisma modifier.
The shard of entropy’s DC uses the creature’s Charisma, and the damage should be no higher than half the creature’s challenge rating in d10s. Lower damage is a good idea for common minions. Only true devotees of Demogorgon would have the maximum shard damage or any sort of field, and no encounter should have creatures with different fields; not only do they take extra time to adjudicate, but their effects stack with each other in absurdly powerful ways.