Because Su Chen's strength has already surpassed the Eternal Ancient Realm, his actual combat power is comparable to that of a Martial Cultivator at the Third Level of the Eternal Ancient Realm, making him the most suitable sparring partner.
With Su Chen's relentless sparring, every time, Old Mu would be beaten until he was almost out of breath, and then Su Chen would offer him a bowl of his own fresh blood.
Using a month of hard effort, he helped Old Mu break through that barrier.
"Since I've already broken through, I can follow Shuilan and protect her at all times from now on. It's time to head to the Primordial Continent," Old Mu said with a laugh.
Why, during this month, have both Su Chen and Old Mu spared no effort to ensure Old Mu breaks through to the Eternal Ancient Realm?
Because of Shuilan.