King of Gwynedd 625-634
Cadwallon ap Cadfan inherited the kingdom of Gwynedd on the death of his father, Cadfan in 625 AD.
However, Edwin who had replaced Aethelfrith as king of Northumbria in 617 AD appeared determined to conquer Britain. Edwin had already eliminated the British kingdom of Elfed and with the death of Cadfan, Edwin invaded north Wales.
Cadwallon managed to escape to Ireland and remained in exile for a number of years, before returning and forming an alliance with Penda, king of Mercia.
Together they launched an assault on the kingdom of Northumbria. They fought Edwin at the battle of Meigen or Hatfield Moor in 632 AD. Cadwallon and Penda were victorious and both Edwin and his son Osifrid were killed; the pair then "ravaged all the land of the Northumbrians" according to the Anglo-Saxon Chronicle.
Bede was sufficiently upset to record the following:
At this time there was a great slaughter both of the church and of the
people of Northumbria, one of the perpetrators being a heathen and
the other a barbarian who was even more cruel than the
heathen. Now Penda and the whole Mercian race were idolators and ignorant
of the name of Christ; but Caedwalla, although a Christian by name and
profession, was nevertheless a barbarian at heart and disposition and
spared neither women nor children. With bestial cruelty he put all to
death by torture and for a long time raged through all their land, meaning
to wipe out the whole of the English nation from the land of Britain.(*)
Although Cadwallon was probably only intent on exacting revenge
for Edwin's earlier invasion
and attempting to destroy the power of Northumbria
and reduce the threat posed by its expansionism.
Whilst Penda returned to Mercia Cadwallon stayed occupied York and undoubtedly carried on with the normal practice of pillaging. The kingdom of Northumbria seems to splintered back into its component parts, with Osric son of Edwin ruling Deira and Eanfrith son of Aethelfrith in Bernicia. In 633 AD Cadwallon defeated and killed both Osric and Eanfrith in separate battles. (Although some say that he simply captured Eanfrith and had him beheaded.)
Cadwallon was himself killed in 634 AD at the battle of Heavenfield or Denisesburn (near Hexham just south of Hadrian's Wall) by Oswald, another of Aethelfrith's son.
Cadwallon was really the last Romano-British ruler to seriously challenge the Anglo-Saxon domination of mainland Britain and those territories that were soon to be known as England. After Cadwallon, Northumbrian expansion continued and the contact between the Britons of the west and those of the north was irrevocably broken.
(*) Interested parties should compare Bede's moral indignation here with the positive glee he exhibits when, for example, the Northumbrian king Aethelfrith slaughtered 1,200 Welsh monks.