The Steam Lord is an enigmatic figure who symbolizes the mystery of both Mechanical and Magical creatures.
No one knows who or what he is, but he has many faces, Many servants, and many frightening powers.
Accepted theories include: A forgotten god, A grudge holding automaton, A Golem, An embodiment of the evils of technology, A ghost. Feel free to add your own.

The main theme here is that he's supposed to be a mixture of Steam-tech, Fancy suit, and some magical influence.
He's also got to look imposing, and not silly.