King of Northumbria (841-844, 844-c848)
Aethelred succeeded to the throne of Northumbria on the death of his father Eanred in 841. Aethelred remained in power for three years before he was ousted by one Raedwulf. Raedwulf however managed to get himself killed by a Viking raiding party within a few months, which left the way clear for Aethelred to return to Northumbria and reclaim the throne.
Around the year 846 Aethelred is recorded as having despatched military assistance to the Picts in their fight against the invading Scots, an indication perhaps that Kenneth Mac Alpin's assumption of power over the Picts wasn't an entirely peacful affair.
Having survived one usurpation however, he failed to survive the next, and around 848 Aethelred was assassinated, either by or at the behest of one Osberht who promptly took his place as ruler of Northumbria.
A Biographical Dictionary of Dark Age Britain by Ann Williams, Alfred P. Smyth and D. P. Kirby (Seaby 1991)
Historia regum Anglorum et Danorum by Simeon of Durham
The Anglo-Saxon Chronicle