The Keepers, who maintain and silently run the Citadel, have gone missing. This has caused great worry amongst the Citadel species because it reminds them just how little they know about the Citadel itself. Where the Keepers have gone is a mystery and why they have gone is even greater a question. The Keepers where the only species who knew the how the citadel truly functions, and it is for this reason that the Citadel races are so concerned with getting them back. Because without the proper maintenance the Citadel could eventually collapse.