Angel (Angel #0)

Meet Angel – a vampire cursed with a soul who has spent centuries protecting humanity from the monsters that lurk in the dark... including himself. Now, looking for redemption for the atrocities committed by the monster he was when he was first turned at the age of 18, Angel stumbles upon a new demon uprising and a shocking truth: The restoration of his humanity can only come if he takes one more life: a certain Slayer from Sunnydale who he’s never met... but may now be his greatest enemy.