From Introduction:
When the wizards of Ardoussarlis delved into another world made of darkness, they found the void sharks waiting, floating in the air. So they plucked these creatures from their home like poison flowers and made dogs of them — vicious hounds to guard their sanctums and hunt their foes. They were well-suited for assassinations and executions, even though the meat of human flesh means nothing to them. For it is the soul they devour. And what ghost was ever made from a soul devoured? None that ever haunted a wizard, no matter how cruel.
In their natural environment, the void shark has no face on the ropy, crescent-shaped organ above their shoulders, and but the one mouth on their abdomen. This face only appears once it has devoured a soul, resembling that of the recently devoured, but with crimson eyes and pointy fangs. This face allows it to see, smell, and taste this world. This face allows it to hunt all the better.
Now that there are no more wizards to command them, the void sharks still hunt at night, but spend their days lounging in the city. In Ardoussarlian times, their summoners would let them loose without fear, for even though their bodies are unaffected by gravity, the air is still poison to them. A mere few days would destroy them. But no longer. The gates between the worlds have let enough of this one into that sea of darkness the void sharks swim through that they have built up a tolerance. The organs inside their bodies are still vulnerable — and all the more so when exposed — but now their skin protects them.