A very nice coin. A lot of history behind this tiny piece.
I found little information of Aethelred,

king of Northumbria. He was son of Eanred and ruled from 841 to 849, with a short period of time when he was expelled in 844 and king Raedwulf came to power. But king Raedwulf was killed in battle in the same year, and Aethelred became king once more. He wasn't a very popular king, because he was assassinated in 849. King Osberht was his successor (849-867).