The origins of the Druids is complex and unclear. So, I shall start at the other end with their demise, which, while still questionable, is at least clearer. According to the Roman writer and historian Tacitus, they were wiped out when a Roman army led by Suetonius Paulinus attacked their base on the island of Mona, (Anglesey, or in Welsh Ynys Môn, just off the Welsh coast).
The truth is more complex, and there is plenty of evidence to show the Druids lasted into the Christian period, most notably in Ireland. But Tacitus provides a good starting point and a notable tale, though of course he was no friend of the Druids, and it was in his interest to glorify the achievements of Paulinus. This is how he describes the battle that occurred, and the massacre that occurred afterwards.
On the shore stood the opposing with its dense array of armed warriors while between the ranks dashed women in black attire like the Furies, with hair dishevelled, waving brands. All around, the Druids, lifting up their hands to heaven and pouring forth dreadful imprecations, scared our soldiers by the unfamiliar sight, so that, as if their limbs were paralysed, they stood motionless, and exposed to wounds. Then urged by their general’s appeal and mutual encouragements not to quail before a troop of frenzied women, they bore the standards onwards, smote down all resistance, and wrapped the foe in the flames of his own brands. A force was next set over the conquered, and their groves, devoted to inhuman superstitions, were destroyed.
They deemed it, indeed, a duty to cover their altars with the blood of captives and to consult their deities through human entrails.
There is an area adjacent to the Menai Strait called the Field of Blood, which may be the place described. It is not relevant here, but while Paulinas was busy in Anglesey, Boudicca was letting rip around what is today London and Colchester, avenging her rape and that of her two daughters.
This could not of course have been the end of the Druids, since many would have been elsewhere, especially in Ireland where the Romans never set food. Nevertheless, it might have been the beginning of the end, significantly weakening the power of the Druids.
The Wisdom of Rhiannon 