Merciless Saviors by H.E. Edgmon

The stunning conclusion to the Ouroboros series, a contemporary fantasy duology in which a teen, Gem, finds out they're a reincarnated god from another world.

That day at the First Church of Gracie changed everything for Gem Echols, and not just because Marian and Poppy betrayed them. Forced to use the Ouroboros knife on Zephyr, who had kidnapped their parents, Gem now has the power of the God of Air.
While for any other god things might work out okay, the Magician-whose role within the pantheon is to keep the balance-having the power of another god has thrown everything into chaos. The Goddess of Death can now reanimate corpses; the God of Art's powers are now corrupted and twisted, giving life to his macabre creations; and, while the God of Land has always been able to communicate with creatures of the Earth, now everyone can hear their cries.
As Gem, Rory, and Enzo search for a way to restore the balance without sacrificing themselves, new horrors make them question how far they're willing to go. In the end, Gem may be forced to fully embrace their merciless nature and kill off their own humanity-if it ever really existed in the first place.
A Macmillan Audio production from St. Martin's Press.