Board Thread:Mortal Kombat X/@comment-173.217.197.6-20140910012332/@comment-21005855-20140918223106

"Tsung's own soul was magically drawn to Outworld, where he was delivered to Shao Kahn. Tsung pledged his soul to the emperor ages before; this agreement was binding, even beyond death. Kahn provided Tsung with a host body and then allied with him in taking his kingdom from  Mileena. Although Tsung is bound to serve Kahn, an opportunity to free himself arises when he discovers Quan Chi is alive. Quan Chi brings word from Shinnok of a plan to take a great power for the  Forces of Darkness, and Tsung realizes if he were to take the power for himself, he could sever his bond with Shao Kahn. Until his opportunity arises, Tsung continues to feign loyalty to the emperor while redirecting suspicion to other members of the alliance."

This whole verse is from Shang Tsung's original timeline storyline.