“They took soot of the furnace, and stood before Pharaoh; and Moses threw it heavenward; and it became boils erupting into blisters upon man and upon beast” (Exodus 9:10). “They took soot of the furnace…erupting [pore’aḥ]…upon man and upon beast” – what is pore’aḥ? It is that they were afflicted with leprosy along with it, as it is stated: “If the leprosy will erupt [paro’aḥ tifraḥ] [in the skin]” (Leviticus 13:12).
“The magicians could not stand before Moses because of the boils, as the boils were upon the magicians, and upon all the Egyptians” (Exodus 9:11). “The magicians could not…” – why could they not stand before Moses? It is because they counseled Pharaoh to cast every son that is born into the Nile, so that Moses would die.9See Shemot Rabba 1:24. Moreover, they condemned him to death because he removed the crown from Pharaoh’s head and placed it on his head.10See Shemot Rabba 1:26.
Therefore, “the magicians could not [stand before Moses because of the boils, as the boils were upon the magicians].” “The Lord hardened the heart of Pharaoh, and he did not heed them; as the Lord had spoken to Moses” (Exodus 9:12). “The Lord hardened the heart of Pharaoh” – when the Holy One blessed be He saw that he did not relent from the first five plagues, from that point on, the Holy One blessed be He said: Even if he wishes to repent, I will harden his heart so that I may exact his entire punishment against him; “as the Lord had spoken to Moses” – as it is written: “But I will harden Pharaoh’s heart” (Exodus 7:3).