Resurgam Trilogy, book 3: Grave
Joan Frances Turner2014

Death was never the end...
With the fragile balance between zombies and the living shattered by an artificial plague, life has only gotten harder for those still walking. While the undead no longer threaten mankind, there are almost no humans left to terrorize: the world belongs to mutation-riddled beings, so-called "exes" that are stronger, faster, and crueler than any human or zombie. For a group of survivors, staying one step ahead of death seems impossible.
Because Death himself has had enough. Cheated of his rightful prizes and taunted by scientists and survivors alike, Death is determined to end it all - permanently.
But even Death has a price.
If you're willing to make the ultimate sacrifice.