The Buddha Emperor, Yin Emperor, and the others only came together after the Buddha Thearch went missing. The Buddha Emperor's powers were already seven-star. He was just one step away from becoming an eight-star martial artist. The Yin Emperor, Night Empress, Xuan Emperor, and the rest were not bad as well. Each of them was six- or seven-star Demigods. They each brought with them a group of Demigods as well.
However, Mei Ru's demonic arts were frightening. Jiang Yi had a taste of it himself. The Buddha Emperor aside—even the Evil Thearch and Beast Thearch might not have been able to fend themselves against her. Thus, Jiang Yi could understand why the entire group of them became her slaves. As for the Buddha Thearch and the others, Jiang Yi guessed that Mei Ru would never have dared to attack them anyway. The Buddha Thearch was a nine-star martial artist after all.
