Demons
As creatures of the abyss, created from Chaos, demons come in many different shapes, sizes, and forms. Because of this they often possess above average strength, sight (including dark-vision), hearing, and speed. Demons may take up different appearances to easily intermingle with the mortal realm. taking said mortal forms halves the demons strength, magic, etc that they have in their natural form. If their disguise does not include wings then they may not fly. In addition, demons may only lie to mortals and other races, to their own kind they may only speak the truth. (half-breeds do not count and one of their own.) Bound to their word.
Demons are inherently evil, good deeds repulse them and thus are of evil alignments (lawful evil, chaotic evil, or neutral evil). The ONLY thing hey see higher than them is the deities. Otherwise demons see them self above all others. A bashful and humble demon rarely exists because of this. A demon's blood is powerful, it has the ability to corrupt, even in half bloods, and as such a full blooded demon may persuade a half breed demon.
Creation Story
Though Chaos was strong, surely the strongest of the gods, the forces and creatures of the worlds were many. Chaos would not abide weakness in any form. A race of clever beings were created which would specialize in each of the other races' weaknesses. They must all be tested, and, if found wanting, destroyed. These beings Compassion named Demons and they excelled in knowing the motivations, tendencies, and most importantly, weaknesses of all living things.
Place in the World
Most demons reside within the abyss where they are free to roam in their natural forms. Often though for various reasons demons will leave the abyss coming to the mortal realms to carry out their own agendas. It should also be noted that demons despise celestials. They cannot help it it is written into their very being. As the natural polar opposite of the demons, often those of the abyss would rather die then breath the same air as their celestial counterparts.