Ragnarök: The Breaking of Helheim (Ragnarök: The Breaking of Helheim #4)

As Thor travels deeper into the monstrous canyons of Helheim, he encounters a wild pack of ferocious wolves, larger than any he has seen before. They bear him a terrible grudge and reveal a hitherto unknown secret of the final battle between the gods and their enemies, a secret that has stretched across the centuries since the fall of the gods. And it could spell death-the real death-for Thor.