Beornwulf deposed Ceolwulf I in 823.
In 825 Beornwulf marched against the West Saxons. Beornwulf's army met them at Ellandun (now Wroughton near Swindon in Wiltshire). The battle ended in a disastrous defeat for the Mercians, and is seen by historians as the end of the so-called Mercian Supremacy. In 826 Ecgberht's son Æthelwulf invaded Kent and drove out its pro-Mercian king, Baldred.
In the wake of these events, Mercia's dominance in southern England rapidly unravelled. Essex and Sussex switched their loyalty to Ecgberht; and the East Anglians asked for Ecgberht's protection against the Mercians in the same year. Beornwulf was killed by the East Anglians in battle while attempting to put down a rebellion.
Beornwulf rebuilt the Abbey of St. Peter (later Gloucester Cathedral) and he presided over two synods at Clofesho (an unknown location believed to be near London with Archbishop Wulfred of Canterbury, in 824 and 825. (Wikipedia).
Coins minted during Beornwulf's reign are very rare, with only around 50-60 known examples.