The Primordials are elemental creatures of enormous power born from the Elemental Chaos. They shaped the world out of the Elemental Chaos and the gods gave it civility and permanence. When the primordials attempted to destroy it to began again the process of creation and destruction, the gods warred with them for control of the new creation. The gods eventually triumphed, and primordials now slumber in remote parts of the Elemental Chaos, rage in hidden prisons, or float, lifeless, through the Astral Sea.
The difference between primordials and archomentals is thin. Some lore suggests archomentals played little or no part in the world's creation and arose at some point afterwards (maybe they are the firsts creations of the original primordials). Most mortals refer to these entities as the Elemental Princes.
When the Abyss was created, the first primordials where drawn by promises of dominion of all the cosmos. Their contact with the plane corrupted them, transforming them into demon lords. Sometime after, the Abyss began to spawn "original" demon lords as well. If the demon lords were ever to join forces, the deities and other immortals of the Astral Sea might not stand against the onslaught. Such an alliance is all but unthinkable, and the lords of the Abyss spend as much time fighting each other as they do countering outside threats.
The oldest myths and legends tell of a race of vile entities that once presided over the remains of a dying universe. These beings of evil incarnate were known as the obyriths. When the gods won the Dawn War, the surviving obyriths retreated into hiding, and many of them pose as demon lords.