As Di Tou had noticed a long time ago, it was typically the second 'objective' on his Mandate's [Guidance] page that correlated and linked to his cultivation progress. Knowing this information, the fact that it required him to resolve his Mental Demon was quite crucial.
After all, the Spiritual Palace Stage was about creating a home in one's heart, and then manifesting it into the 'Void' which was like a false reality. That being the case, considering his Mental Demon was all about where his true 'home' or origin was, it did seem like the two were quite correlated.
