Mind flayers (illithids) began as mysterious, brain‑eating psionic monsters in early Dungeons & Dragons, and over time their origin was expanded into a full cosmic horror story about a fallen empire of alien slavers from beyond reality. Their “true” beginnings now depend a bit on which edition’s lore you follow, but most modern material leans on them as time‑tossed invaders from the Far Realm whose empire collapsed after a slave uprising by the gith.

How did the mind flayer come to be?

Early creation in D&D

  • Mind flayers first appeared in 1975 in TSR’s The Strategic Review as “super‑intelligent, man‑shaped creatures” with octopus‑like heads that attacked with psionic “mind blasts” and devoured brains.
  • At this stage, there was no big backstory: they were just terrifying, tentacle‑faced villains from deep underground, speaking strange languages and treating surface folk as prey.

From underground horror to spacefaring empire

  • In 1st‑edition AD&D they were fleshed out as “illithids,” who hated sunlight and regarded most humanoids as cattle, cementing their role as arrogant psychic overlords.
  • Classic adventures like Expedition to the Barrier Peaks and later Spelljammer lore showed mind flayers piloting nautiloid ships through space, implying they were alien conquerors moving between worlds and crystal spheres.

Lost origin and the gith rebellion

  • Articles such as “The Ecology of the Mind Flayer” and later supplements framed illithids as an ancient, plane‑hopping civilization that had forgotten its original home world after ages of conquest.
  • Their empire eventually collapsed when a slave race—the gith—rebelled, overthrew their masters, and splintered into githyanki and githzerai, who still hunt mind flayers across worlds.

Elder brains, ceremorphosis, and species identity

  • Later editions introduced elder brains: massive communal brains in brine pools that rule colonies and absorb the minds of dead illithids, effectively making the elder brain the “real” continuing entity.
  • Mind flayers “come to be” individually through ceremorphosis, where a tadpole is implanted into a humanoid host’s skull, eats and replaces the brain, and reshapes the body into an illithid under the elder brain’s control.

Far Realm and modern 5e spin

  • 4e firmly tied their cosmic origin to the Far Realm, saying they “came to the natural world from the Far Realm long ago,” making them outsiders from beyond sane reality rather than just deep‑earth monsters.
  • 5e sources like Volo’s Guide to Monsters portray them as a shattered remnant of a once‑dominant empire, perhaps flung into the future, pursuing a “Grand Design” to rebuild their dominion through elder brains, nautiloids, and ceremorphosis.

TL;DR: In‑universe, mind flayers “came to be” as Far Realm–spawned, elder‑brain‑ruled alien slavers whose empire fell to their own former slaves, the gith, while each individual illithid is created by the grotesque ceremorphosis of a humanoid host.

Information gathered from public forums or data available on the internet and portrayed here.