A: A book!
Sorry for the bad joke but the non-corporeal undead also have no bones and one of the best suggested encounter locations was the library, it was just too good an opportunity for a bad joke to skip.
Here we have a rather interesting floor plan of a library. The architecture itself can hint at the former wealth and culture of the city before it fell into decay and abandonment.
We can have the floors littered with a carpet of decaying books and scrolls.
So what the players need to achieve is to find the location of the throne and to learn something of its nature.
I am imagining the library to be abandoned by day but as the sun sets a ghostly presence can be felt.
The monster this time is the former librarian, using the stats of a Wight. These start at 10th level and go up to 20th level. As a single foe against a party of heroes I think that is a fair fight. Given the ability of the undead to regenerate the party may need to fight this wight again and again if they are not adept at getting what they need from the library.
I think it would be important for the GM to describe the wight and it coming for the characters rather than naming it as a wight. If the characters have already fought undead that are hurling shockbolts or lightning bolts and these are doing cold crits it can be one of those situations where the players really have no idea what they are fighting. Most of us started in MERP so wights are nothing new. Against a background of spectres, revenants and apparitions knowing what is what becomes harder.
There is loads of opportunity for atmospheric description here with the sun going down and the characters feeling the temperature dropping. The light fading and then the appearance of burning red eyes in the darkness…
I have not added room numbers to the map intentionally. I think as GMs we can describe a sort of uniform rot and decay. I would suggest that the greyed out areas as the most sodden with swamp water coming up from the ground. the stairs down are flooded with stagnant swamp water. If you need to discourage investigations into the flooded basement we can stick a crocodile down there. Try fighting one of them in the dark. They are only 3rd level but with an 80 OB (Large Bite) and a bit of surprise they could put off the inquisitive!
So now we need to provide the characters with some reward. I do not like the idea of demanding skill rolls to find information that the players need to advance the story. I think rolling to see how soon the information is found is find. I am sure that the RM2 players here probably have Library Use or Research as a skill. I would suggest that good rolls allow for a speedier finding of the right scrolls and books and poor rolls means it takes longer.
Another alternative is that finding the location of the throne is an automatic success but we create a table so that 101+ tells the characters that there is a guardian protecting the throne, 126+ tells the characters that the throne corrupted all who ruled from it. 151+ adds the detail that the throne was known as the necromancers seat and so on.