What's the purpose of Sheng Yuan's descent to earth? What are the conditions?

In my opinion, the purpose of Sheng Yuan's coming to earth with nine heads is to reflect the existence of Emperor Tsinghua of the East Pole (Taiyi saved the suffering Buddha). The Journey to the West's book is based on the historical events of Tang Xuanzang's western expedition to Buddhism. But according to the content of the book, it is actually a novel of gods and buddhas. So it can be said that all the so-called main folk gods and Buddhists are involved. However, from the Taoist immortal system, as one of the six emperors under the Sanqing period, the status of Emperor Tsinghua of Dongji is very high. Judging from the contents of the book, the Jade Emperor, the West Pole Gouchen Emperor and the Purple Emperor Arctic Four Saints all mentioned it. As Emperor Tsinghua of the East Pole at the same level, the author The Journey to the West will certainly arrange certain scenes.

? Emperor Tsinghua of Dongji, also known as "Ten Ways to Seek Sound to Save the Buddha". Taiyi saves the paradise of suffering and lives in Miao Yan Palace, the "Changle World in Tsinghua", where she can extradite the suffering and the dead. For those who accumulate virtue and do good deeds and achieve perfection in virtue, Taiyi saves the god of suffering, "riding the fairy of nine lions and scattering the auspicious light of Wanbao", and then leads them to become immortals. Proficient in three worlds to save suffering (eternal life, human suffering, and death in the underworld). It can not only save lives, but also save lives and benefit all beings. During the Wei, Jin, Southern and Northern Dynasties, he was recognized as the god of the underworld. Its incarnation is Yamaraja, the ten halls of hell.

? Jiutou Sheng Yuan is the Buddha who saved the suffering on Taiyi Mountain, also known as nine lions. Its roar "goes up to three sages and down to Jiuquan", which is also the source of the so-called "lion roar" power in later generations. In the story of Journey to the West, it is a monster that has not completely changed into a human form; At the same time, he is also a monster who doesn't want to eat Tang monk's meat. The reason for the conflict with the Tang Priest's master and apprentice is mainly to take it out on his younger brother (Huang Shijing).

As for the so-called problem, it is because the roar of the nine spirits and three saints can be "up and down" Once fighting with the Monkey King, his voice was heard by Taiyi Tianzun, and he sought to save the suffering by sound as the emperor name of Tsinghua in Dongji. In this way, Emperor Tsinghua of Dongji can stop it in time and recall the mount. So the tricky thing is that Sheng Yuan, the nine spirits, did this on purpose, trying to get the Buddha to take back the Asgard. After all, I came down to earth in private, and I always have to find a step.