The Angel Raziel seems to be behind this whole mess, but we never get to see him in the book, if he even exists. The myth says that his blood, mixed with human blood, and drunk out of the Mortal Cup creates Shadowhunters. So who is this Raziel fellow? He's an international angel of mysteries, like Austin Powers with a holy aura and a less-hairy chest. He was also a pretty kickbutt Playstation game hero in the late-'90s, which isn't as irrelevant as you might think.
We might not know what Raziel looks like, but we get a good look at a few demons in the book. One we hope to never see again is Abbadon, who attacks Clary and friends after she retrieves the Mortal Cup.
Abbadon is a piece of work, isn't it? First it possesses poor Madame Dorothea (that's one thing Abbadon has going for him, if he is a "him": he looks great in drag, even if he does look more like Dame Edna than RuPaul), and then it totally busts loose in all its maggot-infested glory. It's a good thing that demons are severely allergic to sunlight, and that Clary isn't living in Alaska in January, and that Simon can go all Katniss on a bow and arrow.
Abaddon (spelled Abbadon in City of Bones) is the Hebrew word for "destruction" and, as a demon, it takes many different forms. Like a few other of Clare's demons, forms of Abbadon have appeared in multiple Final Fantasy games, and it often looks like a big beetle.
Abbadon, though, is just one of many. The abundance of heavenly and hellish creatures makes us feel like the characters are smaller pieces of a larger struggle that we can't yet understand. Maybe we'll never know. Or maybe we just need to read the next book, City of Ashes.