Dafydd ab Hugh & Brad Linaweaver 
Infernal Sky [EPUB ebook] 

Hawaii— last outpost of civilization on an Earth overrun by demons, traitors. and nightmarish creatures straight out of the pits of hell. Humanity seems doomed to a bloody ending. Then Hawaii receives a message from aliens claiming to be on our side in the battle. Our last chance: make contact. The only man for the job— Corporal Flynn Taggart, U.S. Marine Corps— Fly to his friends. He led the fight against the demon invaders when they swarmed through the Gates at Phobos Base. Now Flys got to face the toughest task of his dirty career. Return to Phobos— and fight his way past those demons to contact mankinds would-be rescuers…