Source: The Office(US)
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The earliest person named in any records as a king of Mercia is Creoda, said to have been the great-grandson of Icel. Coming to power around 584, he built a fortress at Tamworth[citationneeded] which became the seat of Mercia's kings. His son Pybba succeeded him in 593. Cearl, a kinsman of Creoda, followed Pybba in 606; in 615, Cearl gave his daughter Cwenburga in marriage to Edwin, king of Deira whom he had sheltered while he was an exiled prince.
Penda and the Mercian Supremacy
Main articles: Penda, Wulfhere, Æthelbald of Mercia, and Mercian Supremacy
Mercia and the main Anglo-Saxon kingdoms at about 600
The next Mercian king was Penda, who ruled from about 626 or 633 until 655. Some of what is known about Penda comes from the hostile account of Bede, who disliked him both for being an enemy to Bede's own Northumbria and for being a pagan. However, Bede admits that it was Penda who freely allowed Christian missionaries from Lindisfarne into Mercia, and did not restrain them from preaching. Edwin, who had become not only ruler of the newly unified Northumbria, but bretwalda, or high king, over the southern kingdoms, was defeated and killed by Penda and his ally Cadwallon of Gwynedd in 633. When another Northumbrian king, Oswald, arose and again claimed overlordship of the south, he also was defeated and killed by Penda and his allies in 642 at the Battle of Maserfield. In 655, after a period of confusion in Northumbria, Penda brought 30 sub-kings to fight the new Northumbrian king Oswiu at the Battle of Winwaed, in which Penda in turn lost the battle and his life.