The Shadowhunter's Codex by Cassandra Clare, Joshua Lewis Page B

demon’s physical self remains in our world. (Warlock rituals exist that can “preserve” demonic physicality in our world, allowing one to keep, say, a vial of demon blood without its vanishing when its demon source is dispatched.)
WHAT DO DEMONS WANT WITH OUR WORLD? WHY DO THEY COME HERE?
We do not really know. Nephilim folklore says that demons are originally from our world, before they were banished (see Excerpts from A History of the Nephilim , Appendix A), so perhaps they are seeking to take it back for themselves. On the other hand, our stories also tell us that the demons were destructive and wicked from the beginning—that is why they were banished. So they may instead represent the spirit of evil in our world, in some fashion.
At an individual level, demons seem to come to our world in order to wreak havoc. Sometimes they come in search of power—power over other creatures, more powerful magic, and the like—but that power seems to have no ultimate purpose beyond its own existence, other than to wreak more havoc upon our world.
Many have argued for more philosophical origins of the demons’ hate for our world—that they hate our ability to create, for instance, which they lack. This argument is often used to explain why demons create warlocks: It is the only act of creation of which they are capable, and even that they must accomplish by stealing our own act of creation from us.
But we must, for now, throw up our hands and admit that it remains a mystery why demons come to our world. All we can say for sure is that they are here to do us violence, and that they have no interest in truces or treaties.
WHAT DO DEMONS “REALLY” LOOK LIKE? DO THEY LOOK LIKE UGLY MONSTERS EVEN WHEN IN THEIR OWN DIMENSION?
The relationship of demons’ bodies in our world to their “reality” in their own is a secret we may never discover. We believe that demons have no choice about the physical form they take in our world, but other than that, their true form is a mystery. We do not know if terms like “appearance” even apply in the Void from whence they come. One popular theory holds that when a demon travels to a dimension, a body is created for that demon that can survive in that dimension, and that this is the reason that demons are the only creatures that can move freely between worlds. This is mere conjecture, of course.
HOW DO I RECOGNIZE A DEMON IN THE WILD?
Recognizing demons, unless they are shapeshifters, is normally not difficult. Demons always take on monstrous forms in our world, and can usually be discerned by the uncanny, nauseating feeling that billows around them like a dark aura. In the rare cases of uncertainty, positive identification can be made via the demon’s violent reaction to holy items and places or via a Sensor’s violent reaction to the demon.
In addition to their general hideousness, demons often carry with them a scent of death that can be very strong. Shadowhunters asked to describe it usually reach for terms like “rotting,” “spoiled,” “brimstone,” “burning hair,” and the like. The effect can be debilitating to the unprepared Shadowhunter.
    I am actually feeling kind of bad for demons here. Is that wrong?
    Yes.
WHAT IS THAT DISGUSTING BLACK LIQUID THAT APPEARS WHEN YOU CUT THEM?
Like other living creatures, demons’ bodies are kept fresh by a vital fluid, but this is not the usual red blood of our world. Instead, they contain a supernatural ichor. The term “ichor” refers originally to the golden blood of angels, and comes from the ancient Greek word for the blood of their gods. Demon blood is also ichor, but as it is infused with demon energies, it is black and viscous, thinner than blood but totally opaque. Ichor is not dangerous to touch, but it is somewhat toxic to humans if it gets into the blood via a wound or other means. It is unlikely to harm any Shadowhunter who has the usual range of protective Marks, but care should be taken.
CAN DEMONS SPEAK HUMAN