Darth Elmo was a Sith Mega-Super-Overlord. He was known for being more evil than any other being in the galaxy. He was so evil that his attempt to set up his own Galaxy-spanning Sith Empire failed only because he turned on himself. This is what later Sith would enviously call "the ultimate act of treachery."
He was responsible for the training of the infamous Sith Lord Darth Disney who would carry out Darth Elmo's vision of creating a new galaxy.