According to his current theory that’s what happened to Mr. Fur: the memory about how he got free from Tonglu freed his repressed powers and attacked the closest being, according to its instincts. (He Xuan didn’t share it with Harry, as he is sure that in the current state the boy would not react adequately). Weird that his powers didn’t spread further, but maybe he is just too weak "magically" by himself. Or the voice was able to stop it. He Xuan doesn’t like either option, but he would prefer the first one. The least of two evils.