Who is Darius from The Campaigns of Alexander and what is their importance?

Darius was the king of Persia and the leader of the Persian Empire. He confronted Alexander personally in two battles, one near the port town of Issus and another at Gaugamela on the Tigris River. Both encounters proved disastrous for the Persian army, and Darius ended up fleeing from the battle field both times.

Alexander wanted to capture Darius after the second battle at the Tigris River, but Darius escaped with a band of treacherous Persians lead by Bessus. When Alexander drew near, Bessus and the others killed Darius, hoping that they, themselves, would be spared. Alexander, however, considered Darius to be a political equal, although an inferior general, and he avenges Darius by capturing Bessus, humiliating him, and ordering his execution.