Mo Tianyun is the legendary overlord of humanity during ancient times in the Tianyuan Continent. He was the founder of Mercenary City and lead humanity against the magical beasts. He had also repelled the invasion from the War God's campaign after they lost to the Winged Tiger Beast God. He leaves the humans with a legend that glorified his name perpetually, ending with him mysteriously vanishing with the Winged Tiger God into higher planes.
As the story progresses, he is then known to be an even greater expert in the Saints' World, having reached the peak of the martial path long ago. Mo Tianyun is also the great leader of a cult that is feared by many influences there. He is a powerful figure in many realms, having largely affected the development of the Tian Yuan Realm and the Saints' World in the past million years.
It is unknown why he left the World of Immortals. Furthermore, not much is known about his goals other than reviving Ning Shuang through collecting and refining souls all over. This was how the Empyrean Demon Cult was formed, a proxy organization needed to collect as many souls as possible.
He is an ally of Jian Chen yet an enigma in the novel. As the story unfolds, he will be a pivotal character in Jian Chen's rise to power and claim his place as the Chaotic Sword God.
Note: There are prominent differences between the web novel and the manhua.
Jian Chen, the publicly recognized #1 expert in the underworld. His quick sword techniques reached perfection and no one could defeat it. After one battle with the exceptional expert, Dugu Qiubai, who disappeared for a hundred years, his body died and he deceased.
After death, Jian Chen’s soul was reincarnated into a foreign world. He grew up with flying speed, but at the end, because of overflowing enemies and from heavily injuries by them, his soul abnormally changed at the border of life and death. From then on, he stepped onto a completely different path of sword cultivation and became the Sword God of the generation. (By Novelv.com)
There is never an end to cultivation. Saint Emperor is only just the beginning, so it’s nothing to be proud about. We must understand that there is always someone greater than us elsewhere. We have only been sheltered and ignorant in the past..