Why did the last Onmyoji lock Song Zhiyin in the red coffin?

The last Onmyoji, Song Zhiyin, was locked in the red coffin because he was considered to be the most powerful Onmyoji and also an extremely cruel and evil person. He had the ability to control the elements and had once controlled the fate of Onmyoji. Therefore, in order to restrict his movements, the Onmyoji locked him in the red coffin to prevent his power from posing a threat to humans. In the red coffin, Song Zhiyin was imprisoned in a completely dark space. Only a faint light could pass through the coffin's panel. This way, he could maintain his evil tendencies and not interfere with the decisions of the Onmyoji.