King of Northumbria (779-788)
Aelfwald was the son of Oswulf who had ruled in 758-9 and the grandson of Eadberht king in 737-758 and therefore the nearest thing at the time to a 'legitimate' king of Northumbria.
Aethelred, the son of the Athelwald Moll that was responsible for the death of his father Oswulf now held the throne. This was not however, an unsurmountable obstacle as Aelfwald staged a coup in 779 and seized power from Aethelred; it seems he also prudently ensured that a number of Aethelred's noble supporters were killed at the same time. (Although Aethelred himself escaped.) There was also further opposition to his rule from Osbald, an adherent of Aethelred, which ended only when Osbald fled into exile.
As was normal for Northumbrian kings at the time, he reigned for a few years before being murdered by his own patricius, one Sicga on 23 September 788 at Chesters, quite probably at the instigation of the afore mentioned gentlemen Aethelred and Osbald.
After his death he was venerated as a saint which may have been some small consolation. His two sons Aelf and Aelfwine were presumably too young to succeed, as it was his nephew Osred that inherited the throne. (In any case both sons later died at the hand of Aethelred later on in 791.)
SOURCES
A Biographical Dictionary of Dark Age Britain by
Ann Williams, Alfred P Smyth and DP Kirby (Seaby 1991)
Historia regum Anglorum et Danorum
by Simeon of Durham