What did God tell Moses from the beginning about the last plague?
God told Moses from the beginning that the last plague would be super special: it would take the life of everything firstborn. Later He said He would bring one more plague upon Pharaoh and Egypt, and after that one Pharaoh would let the people go.
Les explains that Moses knew God had told him way back at the very beginning that the last plague would be super special. That final plague would be the taking of the life of everything that was firstborn. After Pharaoh angrily told Moses he would not see his face again, Moses answered that Pharaoh had spoken well and that he would not see Pharaoh's face again. Then God encouraged Moses, saying He would bring one more plague upon Pharaoh and upon Egypt, and after that one Pharaoh would let the people go. Les notes that this is exactly what happened.