Invincible - Sherrilyn Kenyon

When teenager Nick Gautier was saved from almost-certain death by a mysterious warrior, he had a feeling his life was set to change. Fighting the undead has taken the place of fighting with his teachers (well . . . almost) and Nick is learning to rely on his powers to protect the world from evil he could never before have imagined.

Then, just when he thought things couldn't get any worse, he wakes up to find himself enslaved in a world of shapeshifters and demons out to claim his soul.

He's being groomed by the darkest of powers and if he doesn't learn how to raise the dead by the end of the week, he will become one of them . . .