Why? Mainly because the Small Domain Master inherently possesses extraordinary combat power, and also holds the Small Domain Master's order.
This is something refined by the Great Domain Masters, who are Immortal Kings, containing a portion of their power, allowing it to easily defeat a Daluo Immortal.
Just like the Green Emperor Order in Mu Rufeng's hand, his strength is too weak to unleash its full power, so Wu Fa cannot slay a Daluo Immortal, but it can still place him in an invincible position.
And once Mu Rufeng's cultivation reaches the Golden Immortal, depending on the Green Emperor Order, he might be able to fight against a Daluo Immortal Peak expert.
It can be said that the Great Domain Masters, the Immortal Kings, have control over the life and death authority and appointments of the Middle Domain Masters, while the Middle Domain Masters control the life and death authority and appointments of the Small Domain Masters.
