He's the Sacred Emperor of the Royal Sacred Sect and Hegemon of the Royal Sacred Region. He was off-world traveling and tempering himself when the sect engaged against Qin Wentian. When things escalated into an all-out war against the Medicine Sovereign Valley, he found that he had already been caught up to by Medicine Sovereign and as a result, called the former Sacred Emperor, Zai Jiang to come and destroy the Medicine Sovereign Valley.
The Sacred Emperor was already an immortal, and these matters were simply beneath his level to care about. Even in the Royal Sacred Sect, it had been many years since he bothered to appear there.