When Pharaoh refused to let Israel go, the Lord brought ten progressive judgments upon Egypt — each surpassing the last, each striking a domain over which an Egyptian deity was thought to preside.
First the Nile was turned to blood. Then the land was overrun by frogs, gnats, and biting flies. The livestock of Egypt died; boils broke out on man and beast; hail mingled with fire destroyed the crops; locusts devoured what hail had spared; and a darkness 'that may be felt' (Exodus 10:21) covered Egypt for three days. Throughout, Goshen — where Israel dwelt — was spared.
The tenth and final plague was the death of every firstborn of Egypt, from Pharaoh's heir to the cattle in the stalls. Israel was commanded to sacrifice a spotless lamb and mark their doorposts with its blood, so that the destroying angel would 'pass over' them. That night became the Passover, the founding feast of Israel — and, in Christian theology, the prefiguration of Christ, 'our paschal lamb' (1 Cor 5:7), whose blood marks the doors of the redeemed.