King Ecbert, who seeks to control Mercia as a vassal-kingdom of Wessex, originally sent an entourage of nobles to accompany Kwenthrith after she was installed as Queen. Ecbert dispatched these nobles for the purpose of monitoring the Queen and providing reports on her actions. However, Kwenthrith subsequently kills the Wessex nobles, rejecting the arrangement with Ecbert and asserting that Mercia is an independent kingdom. In response to this, Ecbert sends a small emissary to meet with Queen Kwenthrith and persuade (or threaten) her to reaffirm Mercia's alliance with and submission to Wessex. Leading this delegation is Ecbert's son, Prince Aethelwulf, who fully believes the mission to be one of suicide.
After failing to seduce Prince Aethelwulf, Queen Kwenthrith has both him and Bishop Edmund forcefully brought into her throne room. She then presents her son, Prince Magnus. When Aethelwulf points out that Magnus is a "Northern name", Kwenthrith proclaims that Magnus is the son of Ragnar Lothbrok. Aethwulf retorts that the Wessex nobles gave no mention of her ever being pregnant, much less that the father was Ragnar Lothbrok. The Queen stands firm, insisting that Magnus is Ragnar's son and that Ragnar will protect Mercia for their son's benefit. She promises that should Wessex attack, they will have to meet with the full force of the Viking army.
Despite the Queen's claims, there is no proof that Magnus is Ragnar's son. Aethelwulf remains unintimidated and dismisses the boy's parentage as irrelevant, telling Kwenthrith "you can't threaten us with him". The Queen eventually agrees to Aethelwulf's demands.
Magnus and his mother Kwenthrith are held hostage in a tower and later saved by Aethelwulf. Later in the mid-season finale, Magnus was once again mentioned by Thorhall, an envoy telling Bjorn and Aslaug about Ragnar's secret about the Viking village being destroyed and the farmer killed. King Ecbert is interested in Magnus to use as a tool for when Ragnar returns to Wessex, seeking to avenge the killing of the Viking settlement. King Ecbert will try to make a deal with Ragnar using Magnus. Ragnar stated that he never had sex with Kwenthrith. Later Aethelwulf banishes Magnus from Wessex in order to save him from a certain death.
Magnus, now a man, returns to Wessex and falsely claims to be a son of Ragnar to Björn and proclaimes he wanted revenge on the new King Alfred, blaming him and the late Ecbert and Aethelwulf for his exile and causing the death of "their father" Ragnar. Magnus, upon meeting Lagertha and Ubbe , encourages them to join with King Harald 's forces, as he is persuaded that King Alfred is tricking them and will not fulfill his promise of granting them lands in East Anglia . However, Ubbe refuses to believe Magnus is indeed Ragnar's son, which Bjorn does. After the departure of the West Saxon army to give battle to King Harald, Magnus goes to the Viking camp and tells him how much he despises King Alfred and that he is a viking. He then joins King Harald and his forces, fighting in the battle of Marton and killing several West Saxon soldiers, including the standard bearer. The viking army is however defeated and Magnus sails away with King Harald and his remaining troops towards York . He professes his faith in the Norse gods and declares that they will eventually triumph over the Christian God, and that one day "the name Jesus Christ will be utterly forgotten", thereby embracing the Viking cause.
|Season three appearances|
|Mercenary||The Wanderer||Warrior's Fate||Scarred||The Usurper|
|Born Again||Paris||To the Gates!||Breaking Point||The Dead|
|Season four appearances|
|A Good Treason||Kill the Queen||Mercy||Yol||Promised|
|What Might Have Been||The Profit and the Loss||Portage||Death All 'Round||The Last Ship|
|The Outsider||The Vision||Two Journeys||In the Uncertain Hour Before the Morning||All His Angels|
|Crossings||The Great Army||Revenge||On the Eve||The Reckoning|
|Season five appearances|
|The Departed Part 1||The Departed Part 2||Homeland||The Plan||The Prisoner|
|The Message||Full Moon||The Joke||A Simple Story||Moments of Vision|
|The Revelation||Murder Most Foul||A New God||The Lost Moment||Hell|
|The Buddha||The Most Terrible Thing||Baldur||What Happens in the Cave||Ragnarok|