The Old Ones were an ancient and highly technologically and psychically advanced intelligent species who established an interstellar empire in the Warhammer 40k Universe over 60 million years ago.
They may have been the first species to attain sentience and the first star-faring species in galactic history. They crafted the Webway, and may be the creators of the Aeldari and many other intelligent races including the primate-like ancestors of Mankind.
They eventually fought a terrible war that they lost and either went extinct or disappeared from the galaxy.