The 3rd Edition D&D sourcebook Lords of Madness says the illithid species originated in the distant future and somehow traveled to the distant past to avoid extinction. A trilogy of 2nd Edition modules says they came from another plane known as Penumbra without the time travel. Has the origin of these creatures been rewritten for 5th Edition, and if so, as how?
[RPG] What are the 5E origins of the Mind Flayers
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Best Answer
Volo's Guide to Monsters (p. 71) offers the most concrete background I could find for mind flayers in 5th edition:
I can't find anything that explicitly says the mind flayers came from the Astral Plane (although the section quoted goes on to theorize that the mind flayers are hiding their empire in the future, so enjoy that reference), but it is implied that the Astral was where they had their original empire. After the war with the Gith, the only survivors were the mind flayers who hid on the Prime Material Plane, thus explaining how and why they're in the various setting worlds.