The Divine Will that Qutong obtained was different from other Divine Wills.
An Xuan noticed that condensing this Divine Will did not consume his own strength. Instead, the Heavenly Dao of the Tianyuan world used him to "copy and paste" the elements of the Seven Kill Calamity, extracting a portion and placing it within Qutong.
It seemed the Heavenly Dao of the Tianyuan world was quite reasonable; it didn't consume An Xuan's own strength, but instead tried it out personally.
Chapter 823