Valkyra: Destiny's Spear — Paul D. Storrie

A brother and sister are thrust into the arcane world of Norse mythology when a spear from the heavens is drawn to them. Now empowered to fight back against this otherworldly evil, the two must evade the monsters who hunt them while trying to get up to speed on the bigger threat to come. The two must entrust their lives, as well as the fate of mankind, to a betrayer who was responsible for destroying the world of the gods and now possibly our own as well.