The emperor sent down an order: every completion levelled cultivator in the Zhou Dynasty can challenge Ling Xian and fight him to death.
After this order circulated around, everyone in the dynasty fell into awe. Especially people like the Third Prince, who wanted to kill Ling Xian himself.
Why?
Because this order was advantageous to Ling Xian. First, this meant only those of the completion level got to fight Ling Xian, which indicated to the original levelled cultivators to not step in. Additionally, he used the word "challenge". This meant they could only openly fight rather than assassinate.
Thus, of course this was advantageous to Ling Xian. After all, with his cultivation level, if he was up against those of the original level, he would be dead.