"The restrictions on the Mental Demon Battlefield were created by the Dark Thearch's Dao Marks. As they were derived from the Heavenly Dao, the power of the heaven worshipping ritual naturally restrained it."
The Dark Thearch was the former Celestial Thearch. Most of his Dao Marks originated from the Heavenly Dao, and his combat power was terrifying like no other.
Even if the former Celestial Thearch had fallen to the darkness, they were still the sealing Dao Marks of a pseudo Heavenly Dao. Even a Myriad Transformations Demon God would have to sacrifice a lot of time and energy to break through them.
Of course, Fang Yuan would not charge straight into it thoughtlessly. He had gathered the Human Dao, caused the human emperor to rebel against heaven, and obtained the support of Excellency Mount Mang. He did not spend a single ounce of energy before he managed to break the seal on the Mental Demon Battlefield.