'It's clear that the king of Northumbria wants nothing to do with pagans. Fortunately, an alternative has presented itself.'
A Tale of Two Kings is a storyline quest that begins after the completion of King of Northumbria and exiting the King's Hall. A man named Wada will approach you and request to speak with you about matters regarding King Osred II. Follow him and do so. In case you haven't followed him, you can find near the collapsed section in the wall
During the conversation with Wada, he will reveal his true loyalty is to that of the previous king, and asks that you help in his reinstatement. No matter what you say, you will be interrupted by Osred's spies and a battle will ensue. After the fight, he will kill all enemies and give you 320 valuables and run off to his king in Yngilwood to wait for you there.