After Mason crosses over into the realm of the Norse gods, Fenn descends into the underworld and races against fate to find Mason and bring her home before she can be transformed into a Valkyrie, which would set the end of the world in motion.
"In this ... conclusion to the Starling trilogy, Mason Starling has taken up the Spear of Odin and has begun the series of events that marks the end of the world"--