Long ago, humans lived side by side with creatures called Guardians. Since Guardians possess powers & abilities that surpassed humans, they were worshiped as Gods. In return, the Guardians maintain the balance of the earth and protect mankind.
Over time, technology took over the role of Guardians, and humans slowly began to abandon their existence. The Guardians then disappeared.
Several millenniums passed, a group of humans accidentally opened the gate that connected Earth to Hell. Creatures from Hell invaded earth and human civilization was crushed. However, just as humanity's hope nearly wiped out, the Guardians returned.