That last door is bugged as hell if you ask me.
I've been through the game several times, and its never opened the first time.
All i can say is keep walking your Ularithid/illithid in and out of the room and up to the door, move him around .. get your Main char. to stand next time him etc. untill the door finally understands that he's supposed to open.
He's allways gotten the point, in time, with my games [img]smile.gif[/img]
Hope it helpz