Offa was King of Mercia, and was mentioned in Season 2 as the father of Kwenthrith, Burgred, and Kenelm of Mercia.
Biography
After King Offa's death, Kenelm took the Mercian throne. Unfortunately, the young king died shortly thereafter and rumors soon started that he was killed by his sister, setting off a nasty civil war as Kwenthrith becomes one of the leading claimants of the Mercian crown.
In History
Unlike the series depiction of Kwenthrith being his daughter, Kwenthrith (Cynethryth) was his wife. They had four children: Ecgfrith, Eadburh, Æthelburh, and Ælfflæd. His rule is seen as one based on increasing his power by conquering smaller kingdoms such as Kent and Sussex.[1]