Quote Originally Posted by Tzardok View Post
IIRC LoM said that Elder Brains absorb the brain and the knowledge of the dead illithids, but the illithids believe erroneously their personality also would become part of it. But that doesn't tell us whether the Elder Brain is created from illithid brains or not.
LoM describes elder brains on page 65, and Tzardok’s recollection is correct: Elder brains consume brains of deceased mind flayers, gaining all their knowledge. Mind flayers believe that their personality survives as part of the elder brain, but that is incorrect.

LoM also states “How an elder brain comes to be is unknown.” So while it is possible that they arise from fused mind flayer brains or have arrived from the same place as the other mind flayers, that is just speculation.

(The whole arrangement with elder brains and mind flayer communities reminds me most of an aboleth and its thralls. So here’s a different kind of speculation: Elder brains are closely related to aboleths – as demonstrated, for example, in their shared ability to preserve knowledge forever and accumulate it without limit – and in this relation lies the source of the great enmity between aboleths and mind flayers.)

Quote Originally Posted by Mr Adventurer View Post
Doesn't the same book also say that Neothelids are what happens when a Mind Flayer tadpole doesn't implant, but survives for a long time without a host?
Yes, this is discussed on page 67. A neothelid can develop when a mind flayer community collapses and no longer feeds the tadpoles and the elder brain in their communal pool. In that case, tadpoles become cannibalistic until the last survivor eventually leaves the pool in search of food. When it consumes an intelligent being, it awakens in the same manner as an implanted tadpole and becomes psychic.