Demons are the Scourge of the world, and the root cause of all the current woes besetting the world.
First coming into the world through the careless manipulation of the Planes, Demons are now stranded on this world, due to the actions of the Cooramoorian Wizards in severing the world from all planes.
The demons stand for nothing other than destruction and chaos. They have no interest in ruling the world, or in enslaving its peoples, all they seek is to see the world destroyed in chaos, flame and despair.
A few brave (or possibly foolish) individuals have investigated Demons over the years, and they have found that there are a number of factions within Demon society, with the strongest ruling over the weaker. Demons will fight amongst themselves if not presented with a more interesting enemy to combat. Jain the Farstrider used this to his advantage when caught by a large Demon during his exploration of the Northern Waste.
Jain recorded his observations of the Demons in his famous book, the Demonomicon. In it he told of the factions that he observed within the Demonic Hordes, and recorded almost all that is known of the Demons. He told of the great Demon Lords, and the battles that he saw fought between them. His stories tell of a society of war and destruction, with no clear overall leader.
During the Last War the Demons demonstrated a degree of tactics and discipline never witnessed before, which enabled them to hold conquered grounds without surging after retreating forces, and even passing by some towns to target more strategically important targets. It would appear that the demons managed to achieve their goal, as after they destroyed Thay they retreated back to the Northern Wastes with little resistance. What they wrought in Thay has not yet been discovered.
It has been theorised that one of the Demon Lords had finally managed to dominate the others, and to focus the Demons into a single force, directed at the Southern Kingdoms. Which Lord was commanding the forces has not yet been determined.
One thing is for certain. A Demon is not open to negotiation, not able to be intimidated, or convinced, and not willing to make a deal. A Demon wants nothing more than to destroy this world and everything in it, up to and including all other Demons, and itself. Until this world is free of the Demons, we will never have rest, and we will never have peace.