Moses didn't send the plagues. God did. But I believe that the death of the first born would be the worst.
After his son died in the Tenth Plague (death of the first-born), Pharaoh "expelled" the Israelites from Egypt (Exodus ch.12), essentially granting the demands of Moses.
We are not give the name of Pharaoh's son who was killed in the 10th plague in Egypt.
The plague of blood in the Bible lasted for seven days, during which time the water in Egypt turned to blood making it undrinkable. This was the first of the ten plagues brought upon Egypt by Moses.
When Moses was in Midian, God sent him back to Egypt in order to begin his mission of taking the Israelites out of Egypt (Exodus ch.4).See also:The ExodusJewish history timeline
I think it was Moses
The first plague was that God told Moses to raise his staff and bang it on the ground near the Nile. This turned all the water in the Nile to blood.
The leader that Moses led out of Egypt was the Israelites.
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God sent Moses to Pharaoh to demand the release of the Israelites from slavery in Egypt, as part of His plan to fulfill His promise to the Israelites and demonstrate His power and authority.
Moses was born in Egypt.