1. A World in the Grip of Monsters
Nearly ten thousand years ago, the last of the human empires fell. They could not stand against the dark forces and magic of the Deep Kings. Whole armies of conscripts were slaughtered or enslaved in the final battles, and fortresses were left without a single stone standing upon another. Now, the good races of the earth are forced into feudalistic serfdom for their inhuman masters. The Dragons, Beholders, Illithids, Liches, Devils, Titans, and other titanic forces of the world rule. Their kingdoms are administrated by their foul spawn, the goblinoids, humanoids, giants, and innumerable lesser monsters. They slay, kidnap, and steal as they will under color of "taxation." The true rulers do not care for the suffering of the people, so long as the production numbers stay high. Towns are walled, but the walls are to keep people in, not out.
2. The Deep Kings
A couple things are required to become a Deep King. One, you must have a large amount of personal force. This is generally represented by being at least a tenth level solo monster, higher than tenth if you're an elite. Normal Monster Deep Kings are practically unheard of (since any normal monster of that height and significance should probably have a template applied.)
The second requirement is that they have to go down, deep. Power radiates from out from the center of the earth. Stone tunnels collect and concentrate it. It disperses swiftly toward the surface. Getting closer to it, and living there, down under miles of rock...that's how you become truly powerful. So, you dig.
But not just straight down. A simple well-shaft isn't enough. You need it to be complicated, with lots of zig-zags, splits, chambers, and mazes. That keeps the power in. So, you build a dungeon on your way down. And you need a way to keep in touch with the surface. So, you get a lot of less powerful monsters to boss around. And they'll come easy, once the Power starts to gather inside you. It lets you lead them, lets you bend them to your will.
I'm not crazy about his gold -> magic -> residuum cycle, but it certainly makes the magic item economy in 4e D&D make a bit more sense. I do think adding a corruption mechanic would be fun. As you push deeper to face the Deep Kings, the corrupting power at the depths of the world begins to infect the PCs, slowly transforming them into the monsters they fight.