Who was the seed that would be a stranger in a land not theirs, and what land was it?
The seed was Abraham's descendants—his children's children—who would be strangers in a land not theirs. That land was Egypt, where God foretold they would serve under bondage and be afflicted four hundred years. This prophecy came in Genesis chapter 15 during the covenant transfer.
Les explains that this prophecy appears in Genesis 15, shortly after God gave Abraham the covenant that out of him would come a nation, a land, and a government with God as King. Abraham asked, "Whereby shall I know that I'll inherit this land?" God descended to man's level and carried out the ancient rite of real estate transfer, so Les calls Genesis 15:12 to the end "Israel's deed." As God deeds the whole Middle East to Abraham, He gives prophecy in verse 13: Abraham's seed, meaning his children's children, would be a stranger in a land not theirs. Les identifies that land as Egypt. They would serve them, go under bondage, and be afflicted four hundred years. Les also broke down that the four hundred years is the sojourning, not just necessarily the affliction, although it certainly sounds that way from the verse.
And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years;