City of Heavenly Fire by Cassandra Clare (The Mortal Instruments #6)

Sebastian Morgenstern is on the move, systematically turning Shadowhunter against Shadowhunter. Bearing the Infernal Cup, he transforms Shadowhunters into creatures out of nightmare, tearing apart families and lovers as the ranks of his Endarkened army swell.
The embattled Shadowhunters withdraw to Idris - but not even the famed demon towers of Alicante can keep Sebastian at bay. And with the Nephilim trapped in Idris, who will guard the world against demons?
When one of the greatest betrayals the Nephilim have ever known is revealed, Clary, Jace, Isabelle, Simon, and Alec must flee - even if their journey takes them deep into the demon realms, where no Shadowhunter has set foot before, and from which no human being has ever returned...