The Book of Destruction
The Citadel “Ghost” is actually an immortal invisible stalker that was programmed to kill all of the clerics during the initial conflict with Talus. The invisible stalker only managed to kill one cleric due to the particular wording of its programming. It was specifically programmed to kill the clerics by throwing them out of the highest window it could reach.
The problem that this poses is not an inability to find windows as they are used liberally in The Grand Citadels architecture, but in an inability to throw the clerics out of the windows. The windows themselves do not open and are protected from harm by the same wards that protect the rest of the building and thus cannot be broken by mortal means. While their are plenty of open balconies that could have served the stalkers purpose it cannot circumvent its programming.
The Citadel Ghost spends its time scraping up the dust remnants of the cleric and throwing the remnants at any and every window it can access. While the leaders of The Grand Citadel are aware of the invisible stalker it poses no threat and is currently just an amusing oddity.