Ahura Mazda is the Supreme God of the "Zoroastrian Pantheon", their Creator God and due to his ridiculous degree of violence and unpredictable shifts in mood, he's given the title of Outer God: "The Unstable."
Ahura Mazda is in the Apocalypse Tournament for two reasons: He was outraged at not being chosen for the Ragnarok Tournament, and also for the Apocalypse's prize of enacting a new law in Heavens, he'll use to declare himself the only one with authority over mortals.