Swarmthief's dance

Fantasy fiction. Long ago, there was war in Heaven. Rann, guardian of the Underworld, fought with the Nulefi, and in that battle the Nulefi were defeated. But though their bodies were destroyed, the spirits of the Nulefi remained, and they wander the world of Myr, cloaked in the shattered form of the Swarms, waiting for the moment of their revenge. Now they have reawoken, and one human boy, Vivreki, has witnessed that rebirth.