King of Northumbria (774-779) and (790-796)
Aethelred was the son Aethelwald Moll who became king by virtue of driving out the previous incumbent Alhred. He lasted until 779 when he was ousted in a bloody coup led by a Bernician called Aelfwald in which a number of nobles were murdered at High Coniscliffe.
However in 788 Aelfwald was himself murdered, and replaced on the throne by Osred II. Aethelred seized the opportunity to reclaim the throne, drove out Osred II and embarked on a systematic purge of his enemies. A number of them (including the sons Aelfwald) were forcibly removed from sanctuary at York and murdered, another rival prince was drowned in lake Windermere, and Osred II executed when he made a reappearance. This was a little extreme even for the eighth century and the Viking raid on Lindisfarne in 793 was widely seen as a judgement on his crimes.
In 791 he married Aelfflaed, the daughter of Offa of Mercia, generally interpreted as an attempt to secure Mercian support for his regime. However it did not prevent his assasination on 28 March 796 by one Ealdred.
Opinions differ on Aethelred; some see him as one of the strongest in a sucession of weak Northumbrian kings and something akin to the 'last best hope' for a revival of previous glories. In reality he was just a brutal thug who succeeded only in further weakening and destabilising the kingdom.
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