The Tenth Plague against Egypt - The death of the firstborn.

Pharaoh's hard heart leads to an unimaginable punishment.

Resources used:

Victor Harold Matthews, Mark W. Chavalas, and John H. Walton, The IVP Bible Background Commentary: Old Testament, electronic ed. (Downers Grove, IL: InterVarsity Press, 2000), Ex 12:1–42.

The Faithlife Study Bible

Sarna, N.M. Exploring Exodus: The Origins of Biblical Israel. Schocken Books, 1986.