King Leoric's Tomb is a sub-level of Level 3 that will appear when the Skeleton King quest is active. In it contains the Skeleton King, which is the remains of King Leoric, and a platoon of skeletons.
After going from the stairway room to the first lobby, there will be two rooms to your left and right. Both rooms will have a lever and some treasure chests with enchanted items. Pulling one lever will open an extension to another room, and pulling another in that extension room will lift the cage that separates the first lobby from the hallway to the second lobby, where the Skeleton King awaits.
In the second lobby is an elaborate structure that's supposed to contain Leoric's body, and three rooms to the front, right, and left. There are corner nooks with Crucified Skeletons, where if you attack enough of them, they will act like a one-way lever and some more room extensions will open up, where you can find extra treasure from chests.
Since the Skeleton King constantly resurrects fallen skeletons (or apparently generates new ones out of the ground on which he stands), they are a constant source of income. Along with the newly raised skeletons come the gold, items, and occasionally even enchanted items. Going at this for many hours can yield you tens of thousands of gold.