[Faceman]

Mind Flayers have the ability to drain Intelligence (Devour Brain). They will drain 3INT each hit (Melee only) and if you have 0 INT you are dead (same goes for strenghth draining or level draining). So if you stay away from them and use missiles nobody will get hurt. The effect is only temporary so wait for it to expire before you go to fight more Illithids.
In BGT (mod) there is a new item (don't know if it is from another mod actually) which negates INT drain. This is overpowered I think as it makes the dreaded Mind Flayer the laughstock of the well-equipped adventurer (as does the original Cloak of Cheese with the Beholder).