The Netherworld is infinitely large and, because time flows unevenly, often looping around, it is infinitely odd.
Magic abounds, making construction and engineering very accessible, but life can be dangerous. Heroes and monsters alike abound, overtaking and invading at the same rate as dungeons, wizard towers, castles, and fortresses are built. History lingers in the air of such places, heavy with the aroma of battle — and sometimes with the ghosts of said!
A company of adventurers will build a headquarters to explore a region, using raw magic to construct a simple structure or two. Through might and cunning, they grow powerful enough to form a guild, then looted treasure is used to fortify these lodgings.
Sometimes one or two adventurers leave the rest as they fall prey to increasingly dark magic, succumbing in the end and becoming the monsters they once fought. Or the rogues of a guild turn into assassins, cutting the names of everyone who once got a cut of the loot. Lacking the combined strength needed to ward off the monstrous forces, someday the guild falls.
The scenarios are endless but the cycle of “build, ruin, reclaimed by nature” is endless — as is the reverse — “monsters build, heroes invade, and reclaim for the forces of light.”
The results are countless layers of abandoned and then forgotten realms to explore. Entire kingdoms have been overtaken by monsters, then knights, then ghosts, only to have the cycle someday start over.