GreatLemur
Explorer
Two people so far have described the aboleth as "silly". I can't really figure out why. They're huge, ancient, tentacled, psionic horrors that turn people into subhuman servitor creatures and--this is the best bit--assimilate the memories of those they consume. An aboleth could send a few skum out to grab the right people, eat some of them for their information, convert a few into skum and then veil them back to their old forms and send them topside, and basically be running the city in a few months. Even the water dependency is cool, because it serves to reinforce their appropriate role as shadowy masterminds. Throw some class levels in there, and you've got a very serious BBEG. And if they're not so "iconic" that the players immediately remember everything they can do, so much the better.
Or, alternatively, you could have an aboleth in a major villain's employ, serving as a very expedient variety of interrogator.
Or, alternatively, you could have an aboleth in a major villain's employ, serving as a very expedient variety of interrogator.