Abyssal Kingdom, Serum Complete

"It seems that we are trapped," Yueha said, her eyes narrowed and somber.

A heavy air loomed over everyone as the group found themselves trapped inside a huge fenced garden with seven dark whirlpools on the ground that oozed out a palpitating aura.

…

The Underwater Abyssal Kingdom was a city-turned-dungeon area, deep below the ocean of this secret realm.

At first, the group entered the ruined city after crossing the barrier and killed monsters there. They found some resources as well, but none that were important to them. 

Since the number of monsters was high, the group took ten minutes to route everything up in the ruined city. After recovering their energies, they moved into the huge castle at the end.

The castle was truly huge, with merely its walls spanning 3 kilometers wide. It was 1 kilometer tall in height as well, its grandness visible at a single glance.