Found, in an Oxfordshire field: the 'lost emperor' who briefly ruled western Europe

When Brian Malin's metal detector emitted a faint beep on a spring evening two years ago, he would have had little idea that his discovery, far from adding a footnote to local history, was about to rewrite that of the ancient world.

When Brian Malin's metal detector emitted a faint beep on a spring evening two years ago, he would have had little idea that his discovery, far from adding a footnote to local history, was about to rewrite that of the ancient world.

Inside the 1,700-year-old earthenware pot that Mr Malin discovered was a small base metal coin that has revealed the existence of a hitherto unknown emperor who ruled Britain and much of western Europe amid dark tales of rape and murder.

The base metal silver "denarius", barely the size of the 20p piece, uncovered by Mr Malin, 32, a factory supervisor from Chipping Norton, Oxfordshire, went on show yesterday at its new permanent home, the Ashmolean Museum in Oxford, 10 miles from the field where it was dug up in April 2003.

The unveiling completed a journey of nearly two millennia for the coin from a Roman mint in Germany to the farmland of ancient England, where its presence confirmed the existence of Domitianus - a forgotten Caesar whose reign in a period of bloody turmoil was so fleeting that he has been called the Four-Day Emperor.

Richard Abdy, the curator of Roman coins at the British Museum, who first spotted the coin as he separated it from a mass of nearly 5,000 found fused together in Mr Malin's pot, said: "It is an exceptionally rare coin. What this small coin establishes is that we now have a new emperor who is not mentioned in the history books.

"His reign, probably because it was so brief, has gone undocumented in his own era. But because of what was found in a field so many years afterwards we can shed new light on a fragmenting Roman empire."

The coin shows a likeness of Domitianus, or Domitian, in profile wearing a crown of rays with an inscription confirming his status as emperor - Imp(erator) C(aesar). It is probably not an accurate portrait, since it is similar to both his predecessor and his successor.

On the reverse is a figure with the words "Concordia Militum". Concordia stands for agreement and harmony and the figure is a traditional statement of loyalty to the army, to whom so many Roman emperors owed their power (and indeed their lives) and from whose ranks Domitianus possibly rose as a general in the praetorian guard.

But rather than being a Caesar anointed by Rome, Domitianus was the ruler of a vast breakaway sector of the Roman empire encompassing modern-day France, Germany, Britain and, initially, the Iberian peninsula. This secessionist Gallic empire separated from Rome AD260 after the emperor Valerian was captured by the Persian king Sapor. The most popular story is that Sapor tricked Valerian into a parley and then used his imperial prize as a human footstool before having him stuffed on his death and displayed in a Zoroastrian temple.

The politics of the breakaway empire, based in the German city of Trier at the heart of Rome's Rhineland stronghold, was no less visceral. A succession of senior army officers installed themselves as emperor, often as the result of the bloody purge of their predecessor, until the Gallic empire was subsumed back into Rome AD274. Official Roman documents from the era described the rulers of the Gallic empire as "tyrants".

The existence of Domitianus was disputed for more than a century after an identical coin to the Oxfordshire find was discovered in a vineyard near Nantes in 1900 and dismissed as a clever fake.

Experts say the fact that Mr Malin's trove, uncovered more than a century later, was fused together proves beyond any doubt that Domitianus succeeded in gaining control of the Roman mint in Trier and struck his own coins - the proof of imperial status. But quite how Domitianus came to don the imperial purple AD271 - and for how long - remains a mystery.

There are two historical sources from the era which refer to a Domitianus. A Byzantine historian tells how a man of that name was executed for treason by the Emperor Aurelian, who ruled from AD270 to 275. Another source states that a high-ranking army officer was involved in a military uprising during that reign.

Mr Abdy said: "It is possible that these documents refer to the figure on the coin but we cannot say that with certainty.

"Because we have records of Gallic emperors who ruled for barely a couple of weeks, it is likely that Domitianus was in control for only a matter of days." Experts believe that the hoard may have been thrown away by its owner because after the fall of the Gallic empire, the coins were worthless.

Mr Malin and the owner of the land where the hoard was found shared a cheque for £40,000, including £15,000 for the Domitianus coin.