Geoffrey tells us how, as the heir of Eudaf Hen's nephew, it was to the young Aldrien that Gwithilin, the Archbishop of London, travelled with a deputation to offer him the British High-Throne. There had been no monarch in Britain since the Emperor Gracian's death, and in 410 the Roman army and administration had decided to completely withdraw back to Rome. Aldrien was aware of the chaotic state of the country, and of the damage incurred from invading Picts, Saxons and Irish. He was, therefore, unwilling to take on the kingdom but sent his brother, Constantine, in his place, with two thousand armed men to secure the throne. Aldrien died some fifty years later, in 464, having married the sister of St. Garmon, Bishop of Man. The couple apparently had a number of sons, including Budic I and perhaps Daniel Dremrudd. He was succeeded by the former.

Aldien only appears in Geoffrey of Monmouth's 'History of the King's of Britain' and is generally considered legendary.