Pharaoh will refuse to let God’s firstborn son, Israel, go, so God will hit him in the heart by killing his firstborn son. Pharaoh considers himself as a son of a god. God will show him that Israel is really God’s son. He kills the son of a god (the crown prince), which is the tenth and ultimate plague.
23 and I say to you, “Let my son go that he may serve me.” If you refuse to let him go, behold, I will kill your firstborn son.’”