God Of Yams

One thing I've always liked from Exalted was the Ebon Dragon (and his opposite, the Unconquered Sun, to a lesser extent). Obviously Obsidian can't just rip content from another setting but I think the idea of a deity of opposition is an excellent idea. An entity who is so opposed to every quest, war, peace negotiation, expedition, or construction that he/she cannot allow anyone to succeed, to the point of sometimes sabotaging her/his own efforts.