When was The Keys to the Kingdom series first introduced?
The Keys to the Kingdom
Nithlings are entities formed from Nothing. They are feared by most of the House due to their ability to kill even an immortal Denizen with a festering bite.
Nithlings take a variety of shapes and are incorporated into every book in some form. They appear to lack complex intelligence, although some types can be used as servants (notably the Fetchers, sent to retrieve the Key in Book One). Most lower forms of Nithling can be dispatched with silver or salt, though this has little effect on higher or more substantially Made Nithlings.