Cyrus was plunged into misery after giving Rita his final command. He could feel his Prime Dantian was getting ripped apart from his spiritual soul.
There exist three Dantian (Lower, Middle, and Upper) in a Spiritual Soul, which could be said is the corporeal consciousness. Dantian were locked treasures in a spiritual soul which could be unlocked by Qi or Magic.
The most common knowledge about Dantian was it could store massive amounts of Qi or magic and continue to increase its capacity with higher realms of cultivation. In simple words, Dantian was the Energy Source Store.
While this was the most basic function of a Dantian.
As for why they were three, it was also because the three types of Dantian were used in different strengthening systems.
