The description of Ioulaum basically satisfies the description of the Adversary. Let us consider:
-Highest form of Illithid lifeform.
-Independent of all other Mind Flayer civilization because created by liches and alhoons.
-Served by the Undead
-Holds the entire knowledge of Netherese Civilization
-Maintains previous personality.
ETA:
Not saying Ioulaum's invovled, but someone in the alliance of the Dead Three, Illithids, and Shar is an unwitting pawn.
I doubt this Ioulaum is around...he was believed dead for nearly 1000 years then was potentially seen once as an undead elder brain with nothing but ghost. Tried to mess with the magic weave but Mystra and another god stopped him. There's also a lot we don't know...how did this guy become an elder brain and had mind flayers serving him? Was he initially an Ulitharid? Did he live with a mind flayer colony for a while? How was he not caught as still being the host in terms of personality? Also given how little D&D did with him, it's doubtful that he is seen as the Adversary internally at WotC. Our player character might become that but given the amount of possible story permutations and endings it seems unlikely that will be D&D canon.
It does make me wonder if half-illithids are a new monster and/or player class that WotC are planning for the next gen player handbook coming next year though.
