In the early Summer of 330, 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. The following description is taken from the Anabasis (section 3.21.6-22.2) by Arrian of Nicomedia; it was translated by Aubrey de Sélincourt.
The death of Darius III
[3.21.6] The news made it plain to Alexander that he must continue to press his pursuit without a moment's delay. His men and horses were both pretty well exhausted already by their unremitting exertions, but Alexander drove them forward none the less, and after covering a great deal of ground during the night and the following morning reached about noon a village where Darius and his captors had stopped the previous day.
[3.21.7] Learning that they had resolved to continue their journey by night, he asked the natives of the place if they knew a short cut by which he could catch up with them. They said they did but the way was through uninhabited country and there was no water - but no matter: Alexander at once ordered them to act as guides.
He knew that the pace would be too much for his infantry, so he dismounted about 500 cavalrymen and mounted in their place the toughest and fittest officers of his infantry and other units, ordering them to keep their own arms and equipment;
[3.21.8] Nicanor and Attalus, who commanded, respectively, the Hypasists and Agrianes, were instructed to take the remainder of the force by the way already followed by Bessus and his party; they were to proceed as lightly equipped as was possible, and the rest of the infantry were to follow in their regular formation.
[3.21.9] 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.note[Near a place called Choara.]
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.
[3.21.10] 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.