First suggestion. Keep the aberrations secret. Keep them safe. None of the common folk should believe that aberrations exist. Giant psychic fish? All a lie, or a case of mistaken identity. Mind flayers? Stories told by dwarven parents to scare their children.
The players should have absolutely no idea what they're in for until the see the illithid, aboleth or whatever for the first time. There should be a lot of build-up and fear before the creatures in question are revealed, first through human cultists or slaves, then from increasingly monstrous servitors.
Include as a secret society both humans worshipping or studying aberrations, and a society of those dedicated to destroying them. The society against aberrations could be a good introduction to one race of aberrations after the players have fought another (presumably weaker), as well as a source of people who actually believe them.
Demiurge out.