In the Dungeons & Dragons fantasy role-playing game, illithids (commonly known as mind flayers) are monstrous humanoid aberrations with psionic powers.

Mind flayers were created by Gary Gygax, who has said that one of his inspirations for them was the cover painting of the Titus Crow book The Burrowers Beneath by Brian Lumley.

[6] Mind flayers first appeared in the official newsletter of TSR, The Strategic Review #1, Spring 1975, in the section named "Creature Features".

[10] The first illustration depicting mind flayers was by artist Tracy Lesch, who envisioned them "like a Ming the Merciless with the mental powers of a Professor X", which appeared in the Blackmoor (1975) supplement.

[18] The alhoon, also known as the illithilich or mind flayer lich, was introduced in the Menzoberranzan boxed set, in the booklet "Book One: The City" (1992).

The book The Illithiad (April 1998[19][20]), and the Monstrous Arcana module series that accompanies it, greatly develops the mind flayer further.

The latter resource, in particular, picks up elements of mind flayer lore from earlier sourcebooks, including The Illithiad [from 2E] and Lords of Madness [from 3.5E].

[32] Christian Hoffer of ComicBook.com commented that mind flayers "have appeared in many 5E adventures over the last decade", however, this module is the first in the edition to have them as the "central villain of the campaign instead of merely one of many side threats.

The most desirable of races for hosts are humans, drow, elves, githzerai, githyanki, grimlocks, gnolls, goblinoids, and orcs.

[35] Alhoons (also called illithiliches) are illithids that choose to focus on developing arcane abilities in addition to their psionic ones,[36] and have grown powerful enough in magic to become undead liches.

[43] Jacob Creswell, for CBR, highlighted that "Vampiric Mind Flayers are a classic Dungeon & Dragons monster that combines two terrifying concepts.

Illithocyte: Illithid tadpoles that survived the fall of a mind flayer empire, they evolved into a new life form and now crawl about in groups seeking psychic radiation on which to feed.

[48] The background material of the Chainmail game[49] places the gith forerunners in a subterranean empire called Zarum in Western Oerik, where they dominated many other races from their capital city of Anithor.

[45] In 2nd edition, they have a second deity named Maanzecorian, who is later killed by Tenebrous (Orcus) in the Planescape adventure module Dead Gods.

Although Ilsensine is the illithid patron deity, few mind flayers actively worship it, thinking themselves the most powerful creatures in the universe.

[24] Currently, the illithids are in a period of intense study and experimentation, gathering knowledge of all sorts that will enable them to eventually reconquer the universe and hold it for good.

They frequently meddle in the politics of other races through subtle psychic manipulation of key figures, not to cause chaos but so as to better understand the dynamics of civilization.

[52] Polygon highlighted that Bluetspur is "a world of cosmic horror populated by malevolent mind flayers that will make your heroes question their own memories".

[53] According to Ken Rolston, the beholder and the mind flayer "win starring roles as intergalactic menaces" in Spelljammer, describing the mind flayers as "evil, brain-sucking horrors who have polished up their social skills sufficiently to present a dubiously neutral facade to trading partners as they secretly scheme toward the day when all intelligent races will be their vassals and brain-food".

[11]: 224–225 In a proposed campaign setting, Pharagos is a lightly populated Earth-like world with three continents and numerous chains of islands.

The one remarkable feature of this world is the presence of an immense petrified corpse of a god previously worshiped by the Forerunner civilization before it was crushed by the illithid empire.

He found them delightful creatures for a sadistic Dungeon Master to use, and a useful bridge between classic game worlds and the planes, as illithids abound in both.

"[62] Screen Rant compiled a list of the game's "10 Most Powerful (And 10 Weakest) Monsters, Ranked" in 2018, calling the elder brain one of the strongest, saying that while the 5th "edition of Dungeons & Dragons has toned down the elder brain a lot", it "still represents a grave threat to most adventuring parties, thanks to its range of powerful enchantment spells and psionic attacks, but it isn't quite the epic level threat that it once was."

"[63] Mind flayers appear in other role-playing games, including Angband, Bloodborne, Demon's Souls, Final Fantasy, NetHack, Lost Kingdoms, Kingdom of Loathing and Lost Souls, and the one-player gamebook RPG series Fighting Fantasy includes a creature similar to the illithid, the Brain Slayer.