The two brothers, together with Harpalus, Nearchus, and Ptolemy, are called "boyhood friends" (hetairoi, literally 'companions') of Alexander, but there are indications that some of them were actually older than the crown prince and served as his advisers. In this quality, they made a crucial mistake when Alexander and his halfbrother Arridaeus reached the age of marriage. Philip and the satrap of Caria, Pixodarus, had arranged a marriage between Pixodarus' daughter and Arridaeus, and Alexander's friends thought that this meant that Philip wanted to make Arridaeus his crown prince. They advised Alexander to send a message to Pixodarus and ask the girl's hand for himself. Understandably, Philip was angry about this proposal, and he sent Erigyius, Laomedon, Harpalus, Nearchus, and Ptolemy into exile.
A year later, Philip was dead and Alexander had become king (October 336). He recalled his advisers and they were rewarded with important offices. Erigyius accompanied Alexander when he invaded Asia (spring 334) and was made commander of a unit of 600 Greek cavalry in the first weeks of 333. In this capacity, he fought at Issus (November 333). He stayed in Syria when Alexander went to Egypt, but was again in the main army during the battle of Gaugamela (1 October 331).
After the destruction of Persepolis (spring 330), Erigyius accompanied Alexander during the pursuit of Darius though Media and Parthia, and is mentioned during the summer campaign to Hyrcania. He was present in Alexander's council during the trial of Philotas, the ill-fated son of Parmenion, and was sent against the rebel satrap of Aria, Satibarzanes. Erigyius commanded this army together with the loyal Persian nobleman Artabazus, the father of Alexander's mistress Barsine. They defeated the rebels in the winter of 330/329; Erigyius killed Satibarzanes in single combat.
After their victory, the two generals joined Alexander at Bactra. Artabazus was made satrap of this region, and Erigyius accompanied the main force to the river Jaxartes (modern Syrdar'ya), where he must have taken part in the spectacular battle against the Sacae. After this, he disappears from our sources. He seems to have fallen ill and died in Sogdia in the winter of 328/327.