Ever since leaving the Devil Abyss, both Jiandan and Mosu had intentionally avoided discussing the sensitive and intimate aspects of their time in the trial Space, each feigning ignorance while knowing the truth full well.
After Mosu and Jiandan left that special Space—especially after Jiandan was struck by two rounds of heavenly tribulation—both understood that the independent realm had been isolated by a mighty being capable of blocking the Heavenly Dao. This meant that Jiandan's so-called Heavenly Dao oath was null and void, yet for the moment, both tacitly chose not to bring it up.
Once Jiandan successfully crossed her Deity Transformation tribulation, Lord Mosu realized he did not have absolute certainty he could subdue her by force. Left with little choice, he could only take a roundabout approach, using the Demon Moon Slash to invite her to Tianmo Palace. Once they arrived at Tianmo Palace, he naturally had ways to resolve his hidden troubles. Being a person of immense pride, he naturally disdained asking others for help.
Chapter 177