Datis and Artaphernes.
He was in his palace at Persepolis in Persia, governing his empire.
Persia had a king, not an emperor. The Persian king Darius I was at home in Persia when the battle of Marathon took place, so he was not killed at Marathon. Darius the Great died of natural causes 14 years after Marathon.
The Battle of Marathon, August/September 490 BC, gave Greece a victory over Persia.
It was Athens against Persia. Athens helped Miletus (another city-state) in a battle against Persia. Athenians didn't actually fight, they just helped Miletus by giving them ships and weapons. Miletus lost against Persia and when Persia discovered that Athens had helped Miletus they wanted the Athenians killed. Thus the battle of Marathon began.
Marathon 490 BCE. Plataea 479 BCE.
The batte of Marathon was between the Athenian people or Athens and the greatest empire of that time Persia.
They were defending their city from an attempt by Persia to instal an expelled Athenian tyrant to rule them.
After the Battle of Marathon, the defeated Persians' punitive expedition returned home and Persia planned a major invasion, which occurred a decade later, and which was also repelled.
Athens/Plataea - Miltiades Persia - Datis and Artaphernes.
Marathon was a plain, not a city. It was owned by Athens. They fought a battle against a Persian amphibious punitive expedition sent by Persia in 490 BCE.
The battle of Marathon was between Athens and its ally Plataia and a punitive expedition sent from Persia in response to an attack by Athens and Eretria on the Persian provincial capital of Sardis in Asia Minor.
An Persian expeditionary force sent by King Darius to punish the Eretrians and Athenians for interfering in his empire was defeated at the Battle of Marathon in 490 BCE.