From Zhong Shenxiu's perspective, this world is coveted by numerous supreme beings, and its defense has cracks.
Periodically, it is polluted by these supreme beings, allowing a large influx of demons through the cracks—this is the truth behind the Heavenly Demon Tribulation!
In fact, the cultivators' resistance is not entirely successful.
At the very least, there are already many Great Evil Demons in this world.
Whenever they strike, even if it's not a full Heavenly Demon Tribulation, it is still a small one.
Moreover, the dangers of this world go far beyond that.
The abstruse Daoist texts, various cultivation techniques, scriptures, etc.—most of them have pitfalls!
Especially the texts of the unconventional paths, many point towards various Great Evil Demons in abyssal lineages, and even to supreme beings themselves.
When a cultivator practices these to the end, they're likely to transform into a demon, becoming traitors to humanity!
