Ziz is a legendary beast of immense proportions, said to be one of the first beings created by the gods, along with the Leviathan and the Behemoth. According to the oldest legends of the Secrets, the Ziz was the first of all the beasts of the sky, created by Oriel. Originally more than one was created, but they were too savage and destructive, leading to the other Deities killing all but one of them to stop them from decimating their fledgling creations. The same treatment was inflicted to the Leviathan and the Behemoth, earning the survivors their common nickname; the "Lonely Three".
Ziz is also famous for having made a deal with Apollion; in exchange for the ability to dream of its fellows, Ziz became the living portal to Heaven. Since the deal, there has been no way to access Heaven other than dying without passing through the maw of Ziz.