Mammon is the embodiment of greed, a prince of hell and the king of one of the 9 circles of Hell. Mammon rules over the circle of greed. His circle is one if the smaller parts of hell but it's all the more lavish. It's kind of like a worse Las Vegas. There you can loose all your life savings behind eery corner. And all the casinos there of course belong to mammon. But in the circle of greed money is worthless. Mammon only cares for gold, rubies and diamonds. Things that have actual value.