He was appointed by King Agilulf in the spring of 591, after Dukes Zotto's death; he was possibly Zotto's nephew.
Arechis was practically independent because his duchy was separated from northern Italy by a stretch of Byzantine territory.
[1] He conquered Capua and Venafro in the Campania and areas of the Basilicata and Calabria.
He spent the last years of his reign establishing good relations with the Roman Catholics of his duchy.
When he died his domain passed to his son Aiulf, despite his recommendation of his adopted sons Radoald and Grimoald.