This was undoubtedly a very experienced assassin.
Had he aimed his first strike straight at Yuan Yang, the ready-to-strike Changhe would have been able to split his head open with a saber.
But he first made a feigned attack on Changhe. During their clash, Changhe naturally shifted his weight backwards, and his sword's momentum was aimed at protecting his vital points. Meanwhile, the assassin had already redirected his effort towards Cui Yuanyang. Changhe was not particularly experienced; had his thinking been even a half-beat slower or his subsequent moves a fraction slower, Cui Yuanyang would have already been lying dead at that moment.
Changhe indeed hadn't managed to adjust his position in time, but unexpectedly, just as the assassin's sword light turned, Changhe's left hand had already flicked a sharp little pebble. It was as if the assassin had turned and presented his back for Changhe to strike, hitting him right at the shoulder well with a "smack".