Ra-den was a very powerful Titanic construct, tasked with leading the Mogu and protecting the Engine of Nalak'sha. After some time Ra-den became disillusioned with his duty and abandoned the Mogu who had been afflicted with the Curse of Flesh.
Stories passed down by the Mogu turned to legend and the Mogu began to believe he was a god. Lei Shen accidentilty discovered him and somehow captured and enslaved him and used his power to fuel the new Mogu empire.
He was the guardian of the engine of Nalak-sha and the Mogu legion. Curse of flesh + no word from the Titans + (unspecified catalyst) prompted Lei-shen to take action against him.