The underworld was situated between heaven and the realm of men. There were various interpretations about its guardian beast, Cerberus. For human adventurers, the name Cerberus meant the beast which guarded the gate towards the underworld for the Gods. Some of the suicidal would go to the gate deliberately in hope that the beast could end their lives in place of themselves. As for the Gods who secretly descended to the realm of men, Cerberus was a brute, and an obstacle set by humans in order to obstruct the gods from going to their realm. Many Gods had battled the beast, but only a few could win.
Apart from Hephaestus the God of Fire, who built the gate towards the underworld, no one elses knew where Cerberus came from. Yet even Hephaestus could only carved two lines of words about it on the gate, which read “A dog of three heads, a gap between life and death.” These words not only failed to provide any clues about the origin of the beast, but also misled people that the underworld was where the souls resided after death, and Cerberus was the guard which protected the souls.
Cerberus’s job was changed since Hades had became the ruler of the underworld. Understanding the true reason behind the existence of the underworld and all the souls, Hades assigned a new task to Cerberus. It was no longer the beast which stopped anyone from walking through the gate. Instead, it joined them. It stepped into the underworld, wandered in the borderless space between different worlds as a traveller of time, and created numerous gateways towards different worlds in the price of its own flesh and blood.
Since then, Cerberus had been protecting the underworld using its own way. It kept transforming itself between a flesh-and-blood body and a terrifying skeleton, until the Gods return from the realm of men one day...