Answer: According to Herodotus, this was Cambyses II.
Explanation: Cambyses II already had a massive empire and then added Egypt to it by defeating Psamtik III at Peluseum in 525 BCE. He soon learned of a rebellion in his home territory of Persia and rushed to respond to it. On the way, he died in what is now Syria. Some sources say it was an accident, others an infected wound. He was succeeded by his brother Bardiya.