Not to suddenly break your eggs, Fred, but I have a doubt/question: isn't Khamûl supposed to be well dead, during the final siege of Dol Guldur? Or, if not 'technically' dead, completely harmless after the undoing of the One Ring and the consequent loss of powers of all who were bound to its might.
Don't get me wrong, I like Khamûl very much. Not as an evil character, but more for his own characterisation. That ability of his is exactly how I generally picture in my mind the properties of the evil Magic sustaining the fortress. Poisonous and slow yet relentless decay, which, in his case, is portrayed as rusting and deteriorating all materials whenever he's near. Only, there is the above-mentioned lore issue to deal with; even if Khamûl had not turned up to aid Sauron in the last moments of the battle in front of Mordor's gates, his prowess would have later vanished either way.
From what we read of Tolkien's words, the memory of Sauron was ever infesting Dol Guldur (that's why it needed to be cleansed), regardless of whom was appointed to be in chief. Aren't the whole poison motive, the Orcs, the other foul creatures and the gaols already sufficient to explore the dreadful aspects of the eerie fortress?