Odin's Ravens (The Blackwell Pages, 2)

Seven kids, Thor's hammer, and a whole lot of Valkyries are the only things standing against the end of the world.
When thirteen-year-old Matt Thorsen and Fen and Laurie Brekke, modern-day descendants of Thor and Loki, discovered they were fated to take the places of the Norse Gods in a battle against the apocalypse, they thought they knew how things would play out. Gather the other descendants, defeat a giant serpent, and save the world. No problem, right?

Wrong. The descendants' journey grinds to a halt when their friend Baldwin is poisoned and Matt, Fen, and Laurie must travel to the Underworld in hopes of saving him. From there, they'll have to reunite the challengers, find Thor's hammer, and save humanity - that is, if they can survive a journey filled with enough monstrous beasts and godly enemies to make Matt and his friends legends in their own right.