City of Heavenly Fire First Synopsis

More City of Heavenly Fire is always better. And here is the first synopsis of City of Heavenly Fire. Damn if that didn't take my breath away...

ERCHOMAI, SEBASTIAN HAD SAID. 

I am coming.
Darkness returns to the Shadowhunter world. As their society falls apart around them, Clary, Jace, Simon and their friends must band together to fight the greatest evil the Nephilim have ever faced: Clary’s own brother. Nothing in the world can defeat him — must they journey to another world to find the chance? Lives will be lost, love sacrificed, and the whole world changed in the sixth and last installment of the Mortal Instruments series!

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 "Must they journey to another world to find the chance?"

Is that "another world" means the heaven? I mean the field of Angels? Because the book is called City of Heavenly Fire... I know that CoHF means redemption. But damn, finding the chance in heaven is so logical to me because things come directly from heaven might be able to kill Sebastian. (And City of Heavenly Fire might have a literal meaning besides Redemption. I mean, City of Bones also have two meanings too. The descend of things and the Silent City.)

Or it can be the Shadow world... kind of thing. It's the place where the souls of dead Shadowhunters rest. Uh... that's a little weird. I wonder what's in it. And if they truly have to travel there, will they be able to meet Will?

Or a more crazy theory. That's the Great City of Pandemonium. So not the best idea on Earth. A field of DEMONS? I prefer deadly glory then. And I see no reason to bust in there unless someone wants to find a more... evil weapon to deal with Sebastian. That's seems a little too wild.