Death’s Shadow by Darren Shan

The apocalypse came and the world burned. But it wasn't the end, and out of the destruction, new life has emerged. Bec is back to face the Demonata. After centuries of imprisonment, she's more powerful than ever, but the demons no longer stand alone.

Something has crawled out of the darkness with her. Lord Loss is no longer humanity's greatest threat...

As Bec's relationship with Dervish improves, werewolves and demons attack and she sends Shark and Meera through a window to another universe to seek Beranabus, and soon all must face an even greater force of evil, the Shadow