Perhaps the visit by the Great Void Sect to the Ancient Longevity World aroused the suspicion of the Emperor of the Dead, or perhaps it was because of some other reasons. Either way, the Emperor of the Dead continued to hide and did not reveal any traces of him.
When the Great Void Sect entered the Purple Mist Sea and investigated till the entrance of the Black Sea, they left soon after they obtained news that the Origins Celestial Dragon had completed his Second Tribulation. Afterwards, there was no news from them. It appeared that they did not wish to further probe into the Black Sea or continue occupying the Purple Mist Sea.
After the Great Void Sect retreated, the Purple Sea Dragon King and the dragons under his command returned to the Black Sea. The other demons did not want to snatch their territories from them.