Dr Chang was an employee of the 3W Institute who maintained the mausoleum inside St. Paul's Cathedral. He met with the Twelfth Doctor and Clara Oswald when they were investigating the facility. Believing them to be government inspectors, he explained the history of 3W, dark water, and created a connection between Clara and Danny Pink inside the Nethersphere.
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After Missy activated the dark water tanks, which in reality contained Cybermen, she told Chang that she was going to kill him and to "say something nice". Despite his pleas, Missy disintegrated him. (TV: Dark Water)