“...[Manasseh] also shall become a people, and he also shall be great. Nevertheless, his younger brother [Ephraim] shall be greater than he, and his offspring shall become a multitude of nations.”
Genesis 48:19

I think "multitude" in this verse is a poor translation. The more literal meaning of "fullness" might sound strange, but I think is more accurate to the original intent. Ephraim, as the representative tribe of the northern kingdom, was scattered into the nations, seeding ideas which would prepare the way for the Good News of Israel's Messiah a thousand years later.