How did Di Renjie assassinate the emperor?

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Episode 16

At this time, the pilgrim activities of Lengguang Temple have begun. The assassin Sha Ping hid on the bigger roof beam and stared at Wu Zetian who entered the hall. His eyes were burning with hatred, but he suddenly hesitated. Suddenly, a white light in the crowd outside the temple rose sharply and went straight to the middle of the back of Wu Zetian. Shaping was taken aback. Hidden weapons instinctively threw it in his hand and shot down hidden weapons who flew to Wu Zetian. The temple was in chaos, and the imperial guards came to protect the emperor, but Shapin took advantage of the chaos to escape. But he failed to escape from Lengguang Temple, which Di Gong expected. Sure enough, General Huan Bin was mistakenly taken hostage by Sharp. Di Gong saved Huan Bin in a crisis, but let the recognized assassin Sha Ping go. Shaping regrets Di Gong's ability, and understands the reasons why the snake spirit has been defeated repeatedly, saying that only Di Gong in the world can make such an accurate judgment. Di Gong asked Sha Ping why he didn't assassinate the emperor, but Sha Ping couldn't explain it clearly. He gave Di Gong a wooden sign with a strange snake engraved on it and his name written on the back: Wenzhong.