He chose Codomannus, a distant relative of the royal house who had previously excelled in a war against the Cadusians (Justin x. 3; cf. Diod. xvii. 5 if.). Codomannus was the son of Arsames, son of Ostanes, one of Artaxerxes's brothers.
The new king took the name Darius, and soon sought to become independent of his assassin benefactor. Bagoas then tried to poison Darius as well, but Darius was warned and forced Bagoas to drink the poison himself.
In 336 BC Philip II of Macedon promised to avenge an old religious offense by the Persians, and sent an army into Asia Minor to "liberate" the Greeks living under Persian control. After taking the Greek cities of Asia from Troja to the Maiandros river, he was assassinated and his campaign, of course, collapsed.
In the spring of 334 BC, his son Alexander the Great began his massive military campaign. In 333 BC Darius himself took the field against the Macedonian king, but was beaten at Issus and Alexander took control of Persia. Then again in 331 BC at Gaugamela, Darius lost control of Babylon.
Arses of Persia
Alexander the Great