Here's the piece from yesterday again.
I don't know where the sloughed flesh look for Orcus started; in 5th edition, I'd guess, but it's actually a good look for him. He looks like he's using his famous wand to add some spice to an elder brain stew, though, which is kinda funny.
The above is another 5e version of the old guy, although from the computer game Neverwinter Nights, not from D&D per se.
These two red-skinned 3e versions of Orcus were commissioned by Paizo for the last adventure path they did in Dungeon Magazine. This was a newish take on Orcus, but not as different as WotC's own approach. In fact, I suspect that Paizo were deliberately trying to return him more to his original vibe after seeing what Wizards had done to him.
Not that that was necessarily a bad look, but when his "classic" look is more like the image below, you can see why Paizo—who are mostly Greyhawk and old D&D grognards and ultra-traditionalists—were trying to "fix" the new direction, if they could.
Some more Wayne Reynolds 4e Orcus
I'm actually not 100% sure that this is meant to be Orcus, but it sure looks like him from the Paizo or 4e version, especially. That big pick-axe he's holding is what makes it perhaps less likely, but being surrounded by undead makes it moreso.
Finally, some 4e Orcus with his big rival, Demogorgon.
OK, I'm just kidding. I have more Orcus in my collection. Might as well pull up a few more.
I can't remember what book this one was in, but I never liked it much. It's a 4e book of some sort.
These two, on the other hand, are unofficial, but at least as good as any official artwork (especially the last one there) but they show the development of Orcus into a red-skinned, bestial-faced creature.
Just for reference, here's his 1e look, as amateurish and garage-band publishing as ever. I admit that sometimes I'm not quite sure what to do with characters like Orcus or Demogorgon. They're interesting, but they're already quite well developed and belong, honestly, to someone else. I suspect that the best way to do so is actually to use the Yog-Sothothery model. By this I mean the following: Clark Ashton Smith did a good job describing what Tsathoggua was in "The Tale of Satampra Zeiros" and he was referred to a bit more obliquely in "The Seven Geases". "The Door to Saturn" and "Ubbo-Sathla." Lovecraft, on the other hand, made even more mentions of Smith's Tsathoggua; referring to him a bit in "The Whisperer in Darkness," "The Mound," "The Horror in the Museum" and even At the Mountains of Madness. But, here he's really only referred to obliquely; the "star" monsters of those stories are not Tsathoggua. He's only mentioned to give the sense of being part of a bigger, secret world of monsters and aliens.
Lovecraft actually did pretty much the same thing with Cthulhu himself, actually. There is one story that prominently features Cthulhu, and that's "The Call of Cthulhu." Otherwise, he's just mentioned. He's referred to obliquely. He appears—kinda—in the background, if at all. And I think that's the best way to utilize Orcus or Demogorgon, really. The names are public domain, the D&D specific details are D&D specific and are not open content.
Of course, in your personal gaming, it doesn't matter. But even there, I think referring to the "famous" characters in the background, rather than making them have guest star appearances, is often a better idea.
That said, there are aspects of both Demogorgon and Orcus that I'd really be keen to use. One of them is their abyssal layers. I actually think both make for fascinating (and obviously very dangerous and exotic) adventure locales that I could see doing something with.
Since I'm talking more about Orcus than Demogorgon, of course, here's a guide to Thanatos that makes for some pretty good summary reading.
EDIT: Minor correction. Orcus is Open Content; he appeared with special dispensation from Wizards of the Coast in Necromancer Games' original Tome of Horrors in the form as he was described in the old Eldritch Wizardry and 1e Monster Manual. Other "classic" D&D demons who so appear are Baphomet, Dagon (in a more mythic looking appearance rather than Lovecraftian), Fraz-Urb'Luu, Jubilex (not Juiblex, although he's identical), Kostchtchie, Pazuzu, and Tsathogga. Not that Tsathogga (yes, they spelled it that way) was a classic D&D demon lord per se, but as I've pointed out multiple times, the Lovecraftian "Great Old Ones" tend to be exactly equivalent to them.
Many of those entries are based on mythological sources, and frankly, many of them don't have a lot of D&D specific development that is unique from the mythological source (like Baphomet, Kostchtchie and Pazuzu, etc.) Others like Fraz-Urb'Luu and Jubilex (sic, or is it?) are inventions of D&D specifically.
It's a shame that they didn't get to add Demogorgon and Graz'zt, and then the demon lord pantheon would be more or less "complete" in open content; those are the only two that really have a lot of D&D specific development that can't be used outside of a D&D context, I suppose.