Shen Mo didn't break his promise. When he refined Yang Chengxiong into a Demon Soul General, he hadn't completely erased his consciousness. Instead, he placed Yang Chengxiong in a higher tier within the banner's Demon Soul General hierarchy. Consequently, much like the Ghost Bride and Master Hong Jue, he possessed greater spirituality than the other Soul Generals.
Shen Mo carefully sensed Yang Chengxiong's lingering obsessions and found that thoughts of resentment and backlash were almost nonexistent. The only strong desires filling his soul were "to devour blood food to strengthen himself" and "to regain full consciousness one day."
With this, Shen Mo felt relieved.
