The name Powys is thought to derive from Latin pagus ‘the countryside’ and pagenses ‘dwellers in the countryside’, also the origins of French “pays” and English “peasant”. During the Roman Empire, this region was organised into a Roman province, with the capital at Viroconium Cornoviorum (modern Wroxeter), the fourth-largest Roman city in Britain. An entry in the Annales Cambriae concerning the death of King Cadell ap Brochfael says that the land later called Powys was originally known as Teyrnllwg.

In 549, the Plague of Justinian – an outbreak of a strain of bubonic plague – arrived in Britain, and Welsh communities were devastated, with villages and countryside alike depopulated. However, the English were less affected by this plague as they had far fewer trading contacts with the continent at this time. Faced with shrinking manpower and increasing Anglian encroachment, King Brochwel Ysgithrog may have moved the court from Caer Guricon to Pengwern, the exact site of which is unknown but may have been at Shrewsbury, traditionally associated with Pengwern, or the more defensible Din Gwrygon, the hill fort on The Wrekin.

In 616, the armies of Æthelfrith of Northumbria clashed with Powys. Seeing an opportunity to further drive a wedge between the North Welsh and those of Rheged, Æthelfrith invaded Powys’ northern lands. Æthelfrith forced a battle near Chester and defeated Selyf and his allies. At the commencement of the battle, Bede tells us that the pagan Æthelfrith slaughtered 1,200 monks from the important monastery of Bangor-on-Dee in Maelor because, he said, “they fight against us, because they oppose us by their prayers”. Selyf ap Cynan was also killed in the battle and may have been the first of the kings of Powys to be buried at the church dedicated to St. Tysilio, at Meifod, thence known as the Eglwys Tysilio and subsequently the dynasty’s Royal mausoleum.

– wikpedia

Æthelfrith of Northumbria was a Chaos warlord, who led his army of brutes and barbarians from the east to destroy the Imperial province of Teyrnllwg. This entire kingdom has fallen to Chaos in modern times, and provides a foothold for those dark forces in the lands of Lyonesse.