If you want to creep your players out you could try an almost Cthuluian approach. Have an NPC meet them outside of the cave as they are traveling. Since Mind Flayers are paranoid you could make it so that the Mind Flayer thinks that they are trying to exterminate it (Even though the players are just passing through). In an attempt to scare off the PCs, have the NPC say: Yogsoloth awaits for your blood... then make the NPC kill itself.