Overlord of the Dungeon (In the System Book #4): LitRPG Series

The Leviathan is dead. Humanity has won the global mission but the price has been too high. Japan is in ruins, Honshu lies partially underwater, and the capital has been destroyed. The country has been left without leadership and tens of millions have perished, while monsters continue to hunt down the survivors.

Ivan Susanin heads to the submerged Toyo as part of an expedition to find the missing raven, help evacuate the civilians and slay as many monsters as possible. And that's only on the surface. He has successfully learned to combine the public good, national interests and his personal goals a long time ago.

But life is full of surprises. A discovered card gives Ivan the right to become Overlord of the Dungeon, an isolated location somewhere on another world. However, he's far from the only contender, and the goblins who inhabit these tunnels have their own plans. Strength alone won't be enough to claim victory this time.