Indeed, there was a rule in the Grand Tournament of the Saint Lands that states that the Lord of a Saint Land could choose to replace himself with whomever that was sent into combat from his corner whenever he pleased. However, there was a condition that mentioned that the Lord of the Saint Land must not fail. Once he did, it would be equivalent to the disqualification of that Saint Land. Therefore, the Lord of the Saint Lands would usually choose to fight in the last battle to be the one to determine their victory. Nobody expected Zhou Weiqing to fight when the tables were turned to the advantage of the Peerless Sect.
Did they really not have anyone else like Tian Yu claimed? However, the lady wearing the hood who killed the Heavenly King from the Blood Red Hell was at least a powerhouse with the cultivation base of a Heavenly Emperor. In terms of power, she might even be on par with Wu Yunyue… So why did they let Zhou Weiqing fight instead of her?