Alexander finds the body of King Darius III

"In the early Summer of 330 BC, Alexander hunted down the Persian king Darius III Codomannus. His courtiers arrested, perhaps because they thought that extraditing him would guarantee their own lives, or perhaps because they wanted to choose a new, stronger king. However, Alexander was too close to deliberate, and when the first Macedonian horsemen appeared, the Persian courtiers decided to kill their king. A relative of Darius, Bessus, became the new king under the name of Artaxerxes V.

Darius was murdered in the desert east of modern Tehran (ancient Rhagae). He was fifty years old.
Arrian:
Alexander himself then started off again at dusk with all the speed he could make, and covering some eighty kilometers in the course of the night, came up with the Persians just as dawn was breaking
They were straggling along unarmed; only a few made any offer of resistance; most of them incontinently fled the moment they saw it was Alexander himself who was upon them. Those who attempted to fight also made off after losing a few men.
Bessus and his friends did not at once abandon the attempt to get Darius away in the wagon, but when Alexander was close upon them, Nabarzanes and Barsaentes struck him down and left him and made their escape with 600 horsemen. The wound proved fatal, and Darius died shortly afterwards, before Alexander could see him.

[3.22.1] Alexander sent Darius' body to Persepolis to be buried in the royal tombs, like the kings before him. [...]

[3.22.2] Such was the end of Darius; he died in July, during the archonship of Aristophon in Athens. In military matters he was the feeblest and most incompetent of men; in other spheres his conduct appears to have been moderate and decent - though the truth may well be that, as his accession to the throne coincided with the declaration of war by Macedonia and Greece, he had no opportunity to play the tyrant."