4 keys and 1 door. Just teleport around until you have 4 darkmoon key. Open the door to enter the beholder level. This level is very tough. So, there are not much puzzle here.
Survival is most important here.
Cast a true seeing spell before entering. Don't heal the beholder. Dragon skin armour. Get the mark of darkmoon at the end of this level. With the mark of darkmoon, you'll may go further.