I remember hearing somewhere that Mind-Flayers perceive time backwards, they're extra-planar in origin, and had an multi-dimensional empire all and they're connected by their psychic link to hive-minds. Their whole make up is supposed to be totally alien to us, like Lovecraftian Elder Gods.
Omeluum doesn't seem to be very alien, in fact I'm surprised they didn't give him more dialogue about Mind-Flayer culture, history and physiology considering this is the first time I can remember having a conversation with one in a D&D game, though I need to replay the Underdark section in BG:2.
