Beorhthun was a dux of the South Saxons.

Bede

Bede

Bede , also referred to as Saint Bede or the Venerable Bede , was a monk at the Northumbrian monastery of Saint Peter at Monkwearmouth, today part of Sunderland, England, and of its companion monastery, Saint Paul's, in modern Jarrow , both in the Kingdom of Northumbria...

's Historia ecclesiastica gentis Anglorum

Historia ecclesiastica gentis Anglorum

The Historia ecclesiastica gentis Anglorum is a work in Latin by Bede on the history of the Christian Churches in England, and of England generally; its main focus is on the conflict between Roman and Celtic Christianity.It is considered to be one of the most important original references on...

(Book IV, Chapter 15) records the invasion of the South Saxon kingdom by Caedwalla

Caedwalla of Wessex

Cædwalla was the King of Wessex from approximately 685 until he abdicated in 688. His name is derived from the British Cadwallon. He was exiled as a youth, and during this time attacked the South Saxons and killed their king, Æthelwealh, in what is now Sussex. Cædwalla was unable to hold the...

 of the West Saxons and the killing of the South Saxon king Æthelwalh. Caedwalla was driven off by Beorhthun and Andhun

Andhun of Sussex

Andhun was an Ealdorman of Sussex under King Æðelwealh, who was slain by the Wessex prince Cædwalla, who invaded and ravaged Sussex. Berhthun and Andhun succeeded in driving Caedwalla from the Kingdom....

who then jointly ruled the South Saxons. However, Bede reports, Beorhthun was later killed and the South Saxons conquered by Caedwalla.