The Dresden Files: Portland
Varies, but usually humanoid. If attempting to appear human, something is always a little “off” about the demonic shell’s appearance—mouth too wide, eyes glow in the dark, claws on the fingertips, etc.
What We Know
Most demons are beings from somewhere in the Nevernever (many hail from a subdomain called the World Below or Downbelow which may or may not be Hell), and they don’t usually pay much attention to the mortal world—unless they’re hungry or just plain mean. Status within demonic hierarchies is important to some; these demons often deal in human souls. Demons don’t" get" sacrifice or love and have no sense of right and wrong. Demons are not the same thing as the Fallen.
Demons in the mortal world are generally either using an ecto-suit construct or possessing a mortal body. Both usually require mortal summoning and power so the demon can cross over from the Nevernever; possession requires substantially more power as far as we understand it. While in theory a demon could cross over without mortal aid, the power requirements to do this from the Nevernever side are staggering—up into the supernatural heavyweight class.
When their shell is destroyed or they are exorcised, demons don’t die—the ectoplasmic body collapses into goo and the animating demonic entity returns to the Nevernever.
Demons may be related tot eh spirits of Rage that empower hexenswolves and lycanthropes and the spirits of Hunger that empower White Court vampires.
Varies; furthermore, demon constructs can be brutal or subtle, smart or stupid, depending upon the particular demon. Like other Constructs, the more power that is available when a demon is summoned, the better and stronger its construct.
For a typical demon, it’s a good bet that they have at least Inhuman Strength, Speed, or Toughness; as more power is placed into the summoning, the remaining attributes get added. If further amounts of energy are available, then each of these attributes can get ranked even higher. Claws and other creature features make frequent appearances, as well as a host of occasional “grow your own” powers.
Can be bound by magic to a wizard or sorcerer’s will, especially through use of their True Name. If control is lost, demons can run amuck.
Running water can ground out a demon’s construct body.
They are usually blocked by thresholds; if they enter without an invitation, they must spend most of their energy just to hold themselves together—that means no freaky magic, just physical capabilities (however, demons possessing a mortal host may be able to work some weird mojo anyway, since they don’t have to constantly work to maintain their integrity).
Demons are blocked by magic circles.
Lastly, they are vulnerable to exorcism (magical and faith-based), the power of faith, and holy objects.