The heartless Pharaoh still refused to free the Israelite slaves. So God, brought about one last plague, which was so terrible that it was certain to persuade Pharaoh to let his slaves go.
That night, God sent the angel of death to kill the firstborn sons of the Egyptians. God told Moses to order the Israelite families to sacrifice a lamb and smear the blood on the door of their houses. In this way the angel would know to 'pass over' the houses of the Israelites. This is why the festival commemorating the escape from Egypt is known as Passover.
This image shows several scenes from Passover. On the right in a domed room, the angel of death is swinging his sword at a man in bed. On the left the Pharaoh and Queen are mourning the death of their first born son. Below is a funeral scene with six men carrying a firstborn's coffin.