Nightlights: Guardians of the Edge

Ajax Holdren didn't think his life could get much worse, until a pretty girl with a sword decided to fight a monster right outside his house. But it turns out the monster is after him, and she's what passes for a guardian angel.

They call themselves Nightlights, and they use the remains of alien technology-- and magical swords-- to fight the living darkness that preys on sentient life. They live in a tower discovered by humanity a thousand years ago via portals controlled by the tower itself. And the very best Nightlights are young adults. Most of them begin the training as children...

Once Ajax meets Natalie, he can't go back, can't pretend the monsters aren't there. The power awakened inside him-- and his own relentless need for a place-- drives him forward. But stepping through that portal takes him into a alien world, where the walls talk and the sun never shines, and humankind's worst enemy isn't the monsters who stalk the street but the people who make them.