A tough area with various undead lurching around such as ghosts, banshees and nightmares. An NPC near the entrance from the Palace Corridor tells you King Cleowyn appears on his throne on special occasions and won't leave until all mortals have left the palace. Another warns that the pit is pitiless, do not go near it, but also says that the traps are dangerous but they may aid you. A third says to obey the King and return his cherished possessions to be blessed. Keep them and you will die.
There are a few locked doors to open with a blue lockpick and others to open by brute force. The pit mentioned is at D5 but I found it no problem; it drops you two floors to the King's Domicile (unless they mean the one in The Mines?). Pillars and trapdoors abound. The trapdoors lead to the Prison on Level 3. Get back by using the pit to the King's Domicile in the Prison and the elevator there to come back up (see those sections).
The trapdoors stop you reaching the bottom left corner of the area - a problem. But a tapestry at K8 tells you that what you see is not always what is real. You'll discover what this means when you approach some pillars and find they move away from you. Use this technique to go through the door at I16 and head along the corridor round the bottom right corner. Go through the door at the end (A10) and the pillar at A8 will move away from you - and keeps doing so. Move along the bottom edge to A5 and then north to B5. You'll enter a major scrap with King Cleowyn and other ghosts! Use Control and Poison Cloud and once you win, you'll gain the King's Ring - very handy.
Return to A5 and the King's spirit will meet you. He tells you the imps that serve Arnakkian have stolen his crown, robe and sceptre. Return them to their proper places in the Mausoleum and he will reward you. He also tells you he was killed by Arnakkian's spirit and you must find his body and put it in the Mausoleum so he can rest in peace. He tells you to find and read the tapestry and the ring is the seal of his promise.
Whatever you do, don't go back to B5. The King will reappear and take back the Key to Cleowyn's Palace before attacking you, necessitating a trek back to the Vestibule for another one.
Instead, head to the tapestry at H12 which was blank until this fight. Now it translates to say that imps have hidden the robe, crown and sceptre. Only a master of the runes may find them. Two-faced Y is at the centre of the quest. From Y, open O and E and I. When a rune key is used correctly, it will disappear. At this stage, this will mean nothing but just wait a little while and all will become clear.
There will still be one small corner, bottom left, that you can't reach due to traps. I assume there is nothing important there.