Behind the city, 100 meters underground where law and ethics have evaporated, lies the circular arena 'Gehenna'.

The wealthy in luxury VIP boxes watch through bulletproof glass as a single human gladiator is deployed, followed by ferocious magical beasts captured from otherworldly rifts. They bet tens of billions on the gruesome sight of a human being torn apart.

The only weapons allowed into the cage are a single dagger or bare hands. However, two monsters reign over that hellish slaughterhouse: Guest and Yoo Chan.

For these two apex predators, who felt a terrible boredom with the weak humans outside who break at a touch, this place is the best hunting ground they sought out themselves and the only playground that makes their blood boil.

Even the organizers dare not control them, offering astronomical fight money, freedom to go above ground, and separate top-floor private rooms, serving them like masters while occasionally gathering strong monsters in an attempt to kill them.