The existence of an Immortal Emperor would distort time.
This was reflected on the River of Time. The "river water" kept raising waves and attacking this Immortal Emperor "lighthouse".
Once the pillar of light that supported the "sun" above was destroyed, the outcome could be imagined.
In a sense, this cultivation method was the true "profound mystery of seizing the creation of the world and invading the sun and moon".
"On the spectrum of time, Immortal Emperors are extremely powerful. They can even attack from the past to the present!" Cui Heng observed the influence of Immortal Emperors on the past, present, and future and was rather shocked.
This also meant that even a Return to Void cultivator like him, if he did not cultivate time-type spells, could be attacked by Immortal Emperors from other space-time dimensions at any time.
Even if he would not be killed, the feeling of being passively beaten would definitely not feel good.