Mona really dug into demonological folklore here, digging up lots of characters from Judeo-Christian tradition and remaking them in the image of D&D. Armies of the Abyss didn't include stats for demon lords, but it included a lot of them as patrons more like gods that could be worshipped by evil characters than monsters to be fought by good ones. This really established him as a guy who knew his demons and knew what to do with them to make them really quite cool.
![lord of chaos lord of chaos](https://images.fineartamerica.com/images/artworkimages/mediumlarge/2/1-lord-of-chaos-pingsan-kates.jpg)
Erik Mona himself penned the demonic counterpart to Legions of Hell: Armies of the Abyss. The demand for more fiends was evident early on in 3rd edition too Green Ronin's Legions of Hell was their best selling product for many years (according to the introduction of Book of Fiends anyway).
![lord of chaos lord of chaos](https://i.pinimg.com/originals/34/48/96/344896d7153c4d8ca9c5c4bdd2cfe0a2.gif)
They also were present from an early stage in the 3rd edition of D&D, although for this edition, we'd have to wait quite a long time for demon lords to make an official appearance. The demons and their lords and their homeland, the Abyss, got an expanded treatment in Planescape, especially in Planes of Chaos and Faces of Evil: The Fiends. Lovecraft, Michael Moorcock, and other writers with whom Gygax or the other developers were familiar (Fraz-Urb'luu, Zuggtmoy, Graz'zt, Llolth, etc.) The demon lords origins are a bit of a mixed bag some are drawn from mythological or Judeo-Christian demonological sources (Orcus, Demogorgon, Dagon, Pazuzu, etc.) while others were invented whole cloth for the D&D game, following in the tradition of Clark Ashton Smith, H. One reason for the demons' memorability is that they are a combination of rank and file demons and big, important personalities in the form of the demon lords.
![lord of chaos lord of chaos](https://i.pinimg.com/736x/42/aa/b2/42aab2869a911123e9720bb9bc7cd454.jpg)
As well as a detailed appearance in the first Monster Manual, they also have a long and storied history in some of the most iconic adventure modules the game ever produced, which is arguably a much more lasting influence than being in a list of monsters anyway. Demons have been around in D&D since the beginning, of course (specifically since 1976's Eldritch Wizardry.) In fact, they've been so prevalent that the game maybe more fairly should have called Dungeons & Demons instead of Dungeons & Dragons, since they're much more likely to appear than dragons.