** spoiler alert ** 
 Sarren is preparing to release a virus that will kill everything -- humans and vampires -- and only three lone vampires can stop him. 
 Of course, it's a bit more complicated than that. "The Forever Song" is a fast-paced, action-filled story full of blood, and darkness, and concludes Julie Kagawa's Blood of Eden series in a thoroughly satisfying manner -- think a balanced blend of vampire/zombie horror, a tinge of romance, and plenty of sword-swinging action in a post-apocalyptic world. 
 Allison, Kanin and Jackal are on the road to find Sarren, battling hordes of rabids and the occasional raider with a sniper rifle. But Allison is beginning to lose herself to the monster inside her, seeing humans only as "bloodbags" -- and it's all because of Zeke's death. As she struggles to keep in touch with her human side, the trio discover that Sarren has taken over Old Chicago. 
 And she discovers that someone she thought was dead... isn't so dead. Zeke is a vampire... and has been brainwashed by Sarren into trying to kill her. But saving him from Sarren's influence is only the start of Allison's troubles, as she finds that she must save him from his own hatred of what he's become. 
 "The Forever Song" pretty much confirms that it pays to set aside teen romance in favor of a vampire killing zombielike "rabids" with a katana. There's still romance, of course -- but the bulk of the book is a wild eruption of gore, steel and shadows. And that is why the book is so arresting, especially the prospect of taking down Sarren and saving the world. 
 Kagawa doesn't shy away from some darker stuff, such as Zeke and Allison temporarily losing themselves to their despair and grief. But she also tries to balance it out with Kanin's lessons about redemption and trying to be the best person you can, despite past mistakes. 
 She also paints a vivid picture of the world dominated by vampires, such a s a Chicago that is now a ruined, rusty shell of its old self. ("Fish glided past us in large schools, flitting through an eerie underwater world of drowned buildings and submerged roads, rusty cars lining the pavement"). And there's some snarky dialogue from Jackal ("They tend to be irrationally paranoid, and his poetry was about to drive me up a wall"). 
 Angsty vampires tend to be tedious characters, but Kagawa handles it pretty well -- Allison's descent into Frenzy and demonic bloodthirst is pretty chilling, although she seems to get past it a bit quickly. And she's backed by a solid string of characters -- Zeke has been broken and left haunted by what he's become, and only has Allison as a lifeline. Jackal is fleshed out somewhat more, and Kanin's dark arc of sacrifice and struggle comes to a satisfying close. 
 After a lot of blood, death and mayhem, "The Forever Song" ends the Blood of Eden trilogy in a very satisfying manner -- and it leaves you eagerly anticipating whatever Kagawa has next in store.