Æthelwold or Æthelwald was the younger of two known sons of Æthelred I, King of Wessex from 865 to 871. Æthelwold and his brother Æthelhelm were still infants when their father the king died while fighting a Danish Viking invasion. The throne passed to the king's younger brother Alfred the Great, who carried on the war against the Vikings and won a crucial victory at the Battle of Edington in 878.
Page from the will of Alfred the Great. The top part, above the 'I', is the end of the preamble describing his councillors' support against his nephews at Langandene. The will itself starts below the 'I'.
Coin of "Alwaldus" (Æthelwold)
Æthelred I was King of Wessex from 865 until his death in 871. He was the fourth of five sons of King Æthelwulf of Wessex, four of whom in turn became king. Æthelred succeeded his elder brother Æthelberht and was followed by his youngest brother, Alfred the Great. Æthelred had two sons, Æthelhelm and Æthelwold, who were passed over for the kingship on their father's death because they were still infants. Alfred was succeeded by his son, Edward the Elder, and Æthelwold unsuccessfully disputed the throne with him.
Æthelred as depicted in the early-fourteenth-century Genealogical Roll of the Kings of England
Charter S 332 dated 863 of King Æthelberht. Æthelred attests second from bottom on the left as "Æthelred fil[ius] reg[is]".
Charter S 338 dated 867. Æthelred, King of the West Saxons and the Men of Kent, grants Wighelm, priest, a seat in St Martin's Church, Canterbury, together with land. Most charters only survive as copies, and this is the only original of Æthelred to survive.