The Children of Wrath by Mickey Zucher Reichert
Tae and his closest friends are charged with breaking the Dark Elves' curse of sterility on the humans of Midguard, with finding the lost shards of the sapphire, the mythical Pica, that might hold the key to the elvish magic. In order to do this he must ensure that Rascal - like him brought up on the streets, still suspicious of all in authority, unpredictable and amoral - plays her part in finding the Pica and does not jeopardise the future of their world. In Valhalla, Colbey Calistinsson, the immortal Renshai, watches and waits. While the humans of Midguard fight their battles, he must ensure that the gods themselves, and Odin in particular, do not shatter the Balance of the Worlds.