In the Great Wilderness Domain, Yin Yang is divided and the Reincarnation Order is established—this is one of the Heavenly Dao Rules. Since it is a new Domain, naturally, new rules and orders must be created.
The existence of the Yin Realm is one of them, and reincarnation is also at the core of the existence of the Yin Realm.
Tian Zi's understanding of reincarnation is ultimately limited; hence, he has not yet formed the Heavenly Dao Rules related to reincarnation.
After Li Xuan broke through to the Creation Realm, within the Great Wilderness Domain, he was capable of establishing the Reincarnation Order, especially since he possessed the Heavenly Dao Seal—establishing the Heavenly Dao Rules for reincarnation was not difficult.
To establish reincarnation, Li Xuan did not use the Heavenly Dao Seal but planned to employ the Daoist Golden Book to establish a powerful reincarnation.