The brooding figure of Penda of Mercia loomed over Oswiu’s rule. Following his defeat of Oswald, Penda became the pre-eminent king in the land. Although he does not seem to have made any serious efforts to enlarge Mercia by incorporating surrounding kingdoms into his own, he was able to remove bordering kings who displeased him, apparently at will. He disposed of three kings of East Anglia, two during their reign and the third in his retirement.

Alongside his exploits as a killer of kings, Penda also forced Cenwalh, the king of Wessex, into exile, as well as repeatedly raiding into Northumbria, at one point laying siege to Oswiu in his Bamburgh stronghold. On that occasion, the prayers of Aidan turned back the flames by which Penda was attempting to burn Oswiu out of his stronghold. But there were other occasions when Penda ravaged the north: Oswiu quite literally bought his kingdom and his life, handing over a vast haul of treasure to Penda. (The Staffordshire Hoard might represent some of this treasure as much of the items appear to have been of Northumbrian origin, although if that is the case quite why the hoard should have been buried and forgotten is a mystery. It wasn’t just treasure Oswiu handed over: Penda took his son, Ecgfrith, as hostage for his father.