In the Exalted universe, the Primordials were the original makers of Creation, but were indirectly betrayed by their oathbound servants, the Gods - starting the Primordial War. In the Age of Sorrows, the Yozis are demon princes, masters of the hell-prison of Malfeas.
The Yozis are cousins to the Neverborn, however, where the Neverborn are dead, the Yozis are alive, but have been bound to Malfeas for all eternity, and can only be released under certain conditions or specific oaths.
They are bitterness incarnate, and seek vengeance against the Gods and Exalted who imprisoned them. Their capacities are generally limited, both by the oaths they swore and the activities of the Exalted, particularly the Sidereal Exalted and their Terrestrial Exalted agents. That said, the Yozis have other methods of interfering with creation, ranging from cultists (like the Lintha family), to their own Infernal Exalted.