Demons

There is no hesitation in my voice as I say that Demons are the most dangerous thing this world has ever seen.
There are many stories about demons in human history. Perhaps the one most known is the idea of the Faustian Bargain. A deal struck between man and demon over the soul of the human. Turns out we were closer to the truth than we thought.
When you are summoning a demon, which I will never recommend you do for the danger it poses to you and those that exist around you for dozens of miles in every direction, you will perform a series of rituals. The first defines a form for the demon. Human, insectile, batlike. Whatever you think fits the task at hand, as long as it has a concrete shape.
Next, you will perform a ritual that develops its contract. The contract must be, and I cannot stress this enough, ironclad. It must clearly describe the demon's tasks, side objectives, the way you want the demon to accomplish its tasks, all the way to its banishment from this plane.
Again, this as well as the form-defining ritual, can not have any room for interpretation. No wiggle room whatsoever.
If the demon manages to work it's way out of the contract, it's abilities become apparent. Not bound by contract or the natural laws of this world, its power becomes infinite. And while its existence will eventually burn out while it strains against reality itself, the damage it can do in the interim is devastating.
The Tunguska Event, is the preeminent idea of a demon off the chain. An entire major city was raised to the ground, in the end. Not to mention the countless small families caught in between the summoning of the demon, and it's final location before it collapses under its own possibility.
It must be stated that we have no proof that Demons, as we know them, come from any sort of "hell" or any sort of afterlife. 
They're not exceedingly talkative on the subject, however.

Basic Information

Anatomy

Undefinable. Form is given by the contract that summons them.
If not bound to a contract, their form is mercurial and frankly impossible to define. Human sight seems to balk at the idea of describing an unbound demon. Fractal patterns and infinitely recursive shapes seem to be the most common image the mind conjures.

Genetics and Reproduction

Unknown. It can't even be determined if they even reproduce in any capacity.

Growth Rate & Stages

Unknown. It can't even be determined if they have any.

Ecology and Habitats

Unknown.

Dietary Needs and Habits

Unknown. Assumed that they do not need to consume anything that could even be confused as real food.

Biological Cycle

Unknown.

Additional Information

Perception and Sensory Capabilities

While it can be assumed they have some sort of sensory perception, as they react to the world around them, the method through which they do so is completely indiscernible to modern science. While they seem to have eyes, they often react to things out of line of sight. They might have ears, but seem to hear even in a vacuum.

Scientific Name

Infernium Possibilitate