Within the cave dwelling.
Su Yu was experimenting on the Fourth Elder Li Zhong from the former Zhaixing Tower, constructing a restriction bit by bit on his Nascent Soul.
This was a divine restriction recorded in the inheritance of a tier five superior Formation Master from Zhaixing Tower. It was the same restriction the Zhaixing Ancestor once set on several Nascent Soul True Monarchs from the Baihua Sect, known as the Celestial Gods Restriction.
Restrictions and formations were somewhat similar, but compared to formations, restrictions were much more exquisite. To set up a restriction, one must master the subtle use of their own Mana, Soul Power, and the power of heaven and earth.
It aligns more closely with the natural Dao power of heaven and earth.
The Celestial Gods Restriction was a type of restriction specifically used on the Soul Power or Nascent Soul, ranging from tier four inferior to tier five superior.