Underworld.
Lin Yuan and Yin Celestial Son and others have returned, while the Heavenly Emperor of Returning Ruins did not follow but stayed within the Returning Ruins.
He gained his path through the Returning Ruins, and now he must remain there to consolidate its power.
This is also why, moments ago in the Void, Lin Yuan did not pursue the Buddha and the Demon Lord.
The Buddha and Demon Lord needed time, and so did they.
Moreover, with ample time, Lin Yuan's preparation would certainly surpass that of the Demon Lord and the Buddha.
Because Lin Yuan holds more worlds than the Demon Lord and the Buddha.
The Returning Ruins is a complete world, in the hands of the Heavenly Emperor of Returning Ruins.
The Mysterious World and the Real World have almost completely merged, which is also a complete world, referred to as Heaven and Earth, under the control of the Yin Celestial Son.
Moreover, the Underworld, evolved from Lin Yuan's tattoos, could also be considered half a world.
