"The so-called 'alliance' is definitely not as simple as verbally agreeing and saying 'from now on, our sides will be together,'" Lord Enma probably wanted to acquire substantial support from Kamiya representing Takamagahara.
"Lord Enma has fallen, only some remnants of his consciousness remain within the Three Paths River. He hopes that you can snatch his Divine Remains Bone from the hands of the Yomi Forces and bring it back to MingHe. Once the Divine Remains Bone is returned, it will merge with the residual consciousness of the great lord and foster the next ruler of MingHe. If you can accomplish all this and bring order back to MingHe, the reborn Enma will bow to Takamagahara and will offer you a generous reward."
Minamoto no Yoritomo stated so.
He was merely responsible for relaying the message, with specifics about the "Divine Remains Bone" and "Second Generation Yama" beyond his full understanding.