A bit offtopic, but most parts of knowledge we have come from somewhere else, do we really need to remember and give credit to everybody? Are we a book that needs bibliography in every post we make? And where do you put the limit? Two lines, two pages?
I consider it polite, when copying text from elsewhere, to at least link to the source. This gives the author some credit, and it also is useful for anyone who is interested in the rest of the walkthrough.
Anyway, the mind flayers you need to kill to complete the quest only appear during the quest. (If you encountered the adventuring party, which also only appears once this quest has started, you're on the right track.) Of interest is that this is one of the few (perhaps the only?) situation where mind flayers appear out in the open (usually, they appear in narrow corridors, so you can just have a mage tank them with Protection from Magic Weapons); however, if you know where they will spawn (or you think you know), you can set up some traps (or trap spells) before the fight actually starts.
Alternatively, if you'd rather skip the whole drow city plotline, just turn the silver dragon hostile; the game will then allow you to leave the underdark. (This is done to prevent the player from being unable to continue the game; in other words, to prevent a softlock.)
Random fact: If you somehow still have the version of Imoen with Imoen's Belt (this is only possible if you manage to glitch past the cutscene where she gets taken away, which is apparently not that easy to do) and are not playing the EE, a mindflayer's Intelligence drain can kill her without triggering the script that makes her leave the party; be aware that she will leave if you cast Raise Dead (but not Resurrection) on her, as the spell leaves her with 1 HP, triggering the script.