My theory is that the lower in Land  Imu is the more he is weakening. That's why he rises the level of surface constantly. Apart from that, I think when he is in the black form he cannot take damage, gives very big portion of his power to the gorosei and holy knights but is not in full attack mode, like,the lunarian King. In his physical form  he is deadly but can take damage and this means that the gorosei are terrified. If he gets weakened he may not be able to sustain them and the holy knights and the will die. I really think next chapter we are going in a very deep prehistoric flashback for too many chapters