Cleon
Legend
This is a bit bare-bones, but it's a start.
Most of it pretty much follows the original text, but it's worth noting that the ability scores of "Str 20, Dex 12, Con 16, Int 19, Wis 17, Cha 17" are a Mind Flayer's plus a size increase to Large. It just seemed appropriate.
I'm wondering about changing the Type to Outsider and maybe tweaking the HD or the AC up a bit. Especially the AC, since AC 17 seems too fragile.
Remember this adventure features rooms with two Sentinel Eidolons (CR 14 each, so a EL 16 encounter) and an Aspect of Ythog-Nthlei (CR 18). There's also an encounter with an Ice Devil and two Horned Devils (CR 13 plus two CR 9, for EL 14). Those are the three toughest encounters I could find with a quick skim through. The Priest encounter
Four of these critters shouldn't be too much of a pushover, although as I suggested earlier they might have been weakened by their long imprisonment or perhaps by being unable to use their "eldritch powers" when encountered because they're using them to break their boss out of prison.
I guess it might help if we decide what EL the encounter ought to be and reverse-calculating the individual CR of the Priest. i.e. if it's an EL 13 then each Priest of Ythog-Nthlei ought to be CR 9.
