Tenebris Purification by James Silvestri (Book 3, the finale)
After a year of humanity dealing with the apocalypse, civilization had successfully adapted to the feral monsters that had accompanied the catastrophic event and the beasts' numbers had begun to diminish.
However, the monsters had been just a prelude to the worst that was to come. One continent had been shattered by widespread afflictions of madness, while another land had been severely affected by an unexpected hardship of its own -- the gods of the Pantheon themselves ruling over the people.
The gods, unable to reconcile with the fact that their fellow trickster god had betrayed them for humanity's interests, rule over their subjects with increasing bitterness and savagery. Humanity, unable to continue accepting these unrelenting otherworldly threats, plots to fight back.
Humanity has little in terms of recourse for dealing with their many supernatural enemies. Everything that they have must be utilized optimally for these final battles -- or else it is humanity that will eventually be cleansed from the world, rather than the gods.