The so-called "alliance" is definitely not just a matter of verbally agreeing, "From now on, we're on the same side"—it's not that simple. Lord Enma is probably hoping to gain some support from Takamagahara, represented by Kamiyagawa.
"Lord Enma has already fallen; only some remnants of his consciousness remain within the Three Paths River. He hopes you can wrest back his former Divine Remains Bone from the Yomi forces and return it to the MingHe. Once the Divine Remains Bone is brought back, it will merge with the remaining consciousness of Lord Enma and give birth to the next master of MingHe. As long as you can accomplish all of this and restore order to MingHe, the newly born Enma will bow to Takamagahara and offer you generous rewards."
Minamoto no Yoritomo spoke these words.
He was only responsible for delivering the message; as for what exactly a "Divine Remains Bone" was, or what "Second Generation Yama" meant, he actually didn't fully understand.