The Devil Sect.
Every story has an antagonist, and when the God's legion began their conquest to conquer the Lower Heavens, they were met by their counterparts, the Devil Sect.
If Ye Tian Yun was the main boss of the Middle Heavens, then the Devils Sect was the main boss of the Lower Heavens which God's Legion had to defeat in order to move forward.
The only difference between Ye Tian Yun and Devil Sect was that this sect was very discreet and never had any clear agenda, on the other hand, apart from the feud which happened between Wang Yao and Ye Tian Yun when they first met, Ye Tian Yun was also after the Emblems which were in God's Legion members' possession.
The Emblems are needed to summon the throne, which will allow its possessor to rule over the Middle Heavens.