So I guess the main question for us is why a Defiler (who really shouldn't read or dress like that unless he wants to die of heatstroke or lynch mob), a Thri-Kreen (who are carnivorous and get their fluids from eating animals and people), and an undead (HA, YOU PUNY MORTALS AND YOUR BIOLOGICAL NEEDS) are tagging along on a mission to find a hidden underground oasis. Athasians, particularly level 3 ones, don't have time for flights of fancy like an oasis only know of from rumors: they need to SURVIVE. As level 3 characters, we're the equivalent of the typical dirt-farming peasant from Greyhawk or Faerun.

Best I can figure for the undead is he knows if you bite it he can drag your stuff back to town and sell it, and he can body enough of the mind-affecting psionics from the wildlife to find his way back.