What if the original 19 kings never died?

Imu is an entity and carries the souls/wills of the 19 founders and can place them into vessels like the Holy Knights or Gorosei.

​

That would explain why Harald's death seemed to affect Imu so much: one of those ancient souls may have been lost permanently.

​

Basically, Imu is a mix of Horcruxes and Pain's network from Naruto.