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 Je te dis: Laisse aller mon fils, pour qu'il me serve; si tu refuses de le laisser aller, voici, je ferai périr ton fils, ton premier-né.