The Last Army of Man by R A Dukes

Portals to hell have opened all across the world, releasing demons into the human world. The demons attack and slaughter the majority of humans, leaving only small groups struggling to survive and looking for safe havens. This is the story about one group that actively seeks out humans and provides a safe haven, at a cost. Kovik, who despises humans, with good reason,unknowingly starts down a path that will lead to mankinds salvation, or final destruction.