Agatheron's Points of Light
Creations of the great yawning Abyss in the heart of the Elemental Chaos, Demons are creatures of pure malevolent destruction. While they share similarities with the elemental creatures of the chaos, demons go far beyond the untamed nature and are engines of annihilation. The most powerful of the demons are the demon princes, who were once thought to have been powerful primordials in their own right, who were irretrievably corrupted when drawn into the swirling mass of the Abyss.
Even then, many of the demon princes themselves fear venturing too far into the heart of the Abyss, for they know that pure annihilation is at its heart. So they create realms of their own, to gather strength so that they might carry out their wanton destruction of their own. Mercifully for the rest of the cosmos, the very nature of the demons prevent them from uniting as one force, for if they had then the annihilation that the Abyss embodies would be realized.
Most of what characters may encounter of the demons are small embodiments of malevolent entropy, and rarely do they encounter the true horror of what demons are capable of. However, even this small glimpse is more than enough.
It should be noted that with the exception of Lolth and Gruumsh, all of the gods, good, unaligned or evil never employ demons in their service and remain vigilant of the inhabitants of the Abyss, seeking to thwart them at all costs.