Kwenthrith was the seductive Queen Regnant of Mercia.
Kwenthrith was a Princess, and the daughter of the late King Offa of Mercia. She was also the sister of the late Prince Kenelm, who's since been sainted by the pope, and Burgred, who was fighting with her uncle, Brihtwulf, to prevent Kwenthrith from claiming Mercia as her own.
As a child, Kwenthrith was raped by her uncle Brihtwulf at the age of 6. Brihtwulf would also bring other men to her so that they might, as she puts it, 'enjoy the pleasure of having sex with a child'. Her uncle also forced her to have sex with Kenelm when she was 12.
After the death of their father, 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 became one of the leading claimants of the Mercian crown.
Kwenthrith is introduced arriving at King Ecbert's castle. He realizes the advantage of influencing Mercia's future, and seeks to ally himself with the princess, at least for the time being. He welcomes her to Wessex with a grand feast, where she engages Athelstan in conversation about his experiences among the Northmen. Kwenthrith proceeds to ask the flustered monk about the liberal sex life of the Northmen, and how much more "natural" it seems.
She later shares King Ecbert's bed, though he is unfortunately finished (and quite exhausted) before the princess has had enough. The king sends in three of his guards (who are happy to oblige) in an attempt to satisfy her seemingly insatiable sexual appetite.
When King Ecbert hires a company of Vikings as mercenaries, after a stunning victory against them in battle, the princess strolls through the ranks, remarking that she would like to breed with them and would certainly produce "giants" as offspring.
When Ragnar returns to Wessex, King Ecbert asks him to fight for Princess Kwenthrith so that she may claim Mercia and the crown. Ragnar is uninterested in helping her and tells the King that this was not part of their treaty. Nevertheless, he agrees to help.
Not one to sit idly, Princess Kwenthrith joins Ragnar and his warriors as they seek out her brother and uncle. She promises Ragnar that she can offer him something special should he follow through in his task. The Northmen find the Mercians with large armies on each side of an expansive river; Ragnar decides to take on her uncle, confusing Burgred who does not understand why they are not attacking both sides. A bloody battle ensures, leaving Brihtwulf dead.
Kwenthrith, heavily intoxicated and wildly pleased with her uncle's death, asks Ragnar for his head, to which he agrees. Ragnar learns that her uncle used to rape her and "shared" her with other men and her brother, Kenelm, never relenting despite her cries. The princess happily takes the head and stabs it over and over again, finally free of him.
Ragnar safely recovers her brother, Burgred, as promised. Ragnar warns her that her brother is weak but could still cause her downfall; Kwenthrith takes little notice of this and instead focuses on a wound Ragnar received during battle. Kwenthrith decides to urinate on it to help sterilize the area.
At a grand feast held by King Ecbert, Princess Kwenthrith decides that her brother is no longer needed - she poisons his drink and he subsequently dies, which causes all present to pour out their drinks in a rather macabre toast. It is clear that the now-Queen will be no one's pawn and fully intends to take Mercia for herself. She decides to rule Mercia alone, breaking her arrangement with King Ecbert and killing the Wessex nobles that had been sent to monitor her. The king sends his son, Aethelwulf, to Mercia to try to reason with her. She uses her charm and sex to try to take advantage of Aethelwulf, though he manages to rebuff her. She reveals to him instead a baby boy, Magnus, whom she says was fathered by Ragnar. Kwenthrith claims that Ragnar will protect her and her son, though Aethelwulf does not believe her: none of the now assassinated Wessex nobles had ever reported that she was with child.
Aethelwulf informs Kwenthrith that he is fully aware that his father is using him and acknowledges that he will likely be killed, his father then using his death as a pretext to invade Mercia. This threat seems to chasten Kwenthrith and she accedes to Aethewulf's demands, who returns to Wessex.
Unfortunately for Queen Kwenthrith, the nobles of Mercia have revolted against her and she has been taken prisoner. News of this reaches Wessex, and King Ecbert learns that Kwenthrith and her young son Magnus have been locked in a tower. Ecbert sends scouts to Mercia to locate the captive Queen and, although most are killed, one man is able to return and report to the king where the Queen is being held. Ecbert sends his son, Aethelwulf, to rescue them.
In Mercia, Aethelwulf and his men find the queen and her son, though a battle soon breaks out and the Mercian general commands his men to kill the Queen. Kwenthrith fights desperately to save her life and manages to fight off the men with the help of Aethelwulf.
Returning victorious from Mercia, Kwenthrith quickly invites Aethelwulf to her chambers. This time, he accepts her offer. Meanwhile, Ahtelwulf's wife, Judith, has become the mistress of King Ecbert.
King Aella arrives in Wessex, his mind heavy with the issue of Mercia. While at dinner, Aella notices something is amiss as Aethelwulf cozies up to the deposed Queen Kwenthrith while Judith giggles with Ecbert. Aella confronts his daughter about her scandalous relationship: "You don't own me, father", Judith tells him. The marriage is long over - Judith refuses to sleep with her husband, despite his threats.
Martial problems aside, the issue of Mercia and of Kwenthrith still hovers over the minds of the Kings. Naturally, Aethelwulf sides with Kwenthrith who still wants to rule over Mercia. Ecbert agrees and decides to march with an army to Mercia, meanwhile sending Aethelwulf to Rome as Alfred's escort. When Ecbert returns, having killed the ruling council members, Kwenthrith believes she is to return home and rule. She is also excited to announce her pregnancy by Aethelwulf. Unfortunately, her happiness is short-lived: before killing the councilmen, Ecbert has them sign documents proclaiming himself as king.
Realizing she is once again a captive, Kwenthrith turns to Judith for help escaping the castle. Kwenthrith admits that Aethelwulf is the father of her child, and Judith forgives her. Unfortunately, Judith can not help her and only watches as Kwenthrith makes a failed attempt to escape with her son. Kwenthrith will not be held prisoner again; however, and says goodbye to her son before heading towards Ecbert's chambers. She stabs a soldier (Waerferth) who attempts to stop her before making it to Ecbert. Holding a knife to his throat, Kwenthrith ignores his warning that she will be killed by his guards, but is stabbed in the back by Judith before she can kill the king. Kwenthrith falls on the bed, telling Judith that she has "killed twice over" before succumbing to her wound.
- Kwenthrith appears to be based on a Mercian princess by the name of Cwenthryth. Unlike the series depiction of her, Cwenthryth was the daughter of King Coenwulf (reigned 796–821 AD), distant cousin to King Offa of Mercia (reigned 757-796 AD). Cwenthryth was the reigning Abbess of Minster-in-Thanet in Kent, which was a source of contention between King Coenwulf of Mercia and the Archbishop Wulfred of Canterbury, over the issue of whether religious houses should be controlled by clergy or lay-nobility. Cwenthryth was, however, the sister of Saint Kenelm.
- The name Cwenthryth (Kwenthrith) likely means 'woman of power'.
- There was a daughter of King Offa of Mercia called Eadburh  who Kwenthrith also appears to be loosely based on. Eadburh (Old English: Ēadburh), also spelled Eadburg, (fl. 789-802) was the daughter of King Offa of Mercia and Queen Cynethryth. She married Beorhtric, king of Wessex from 787 to 802, in 789. Offa was then the most powerful king in England, and Beorhtric gained his support as a result of the marriage. According to Asser, Eadburh became all powerful, and often demanded the executions or exile of her enemies. She was also alleged to have assassinated those men whom she couldn't compel Beorhtric to kill by poisoning their food or drink. In 802, according to Asser, Eadburh attempted to poison a young favorite of the king but instead killed both of them. The young man may have been called Worr, as the Anglo-Saxon Chronicle records the death of both men shortly before the succession of Egbert, the grandfather of Alfred the Great, as king of Wessex'
|Season two appearances|
|Brother's War||Invasion||Treachery||Eye For an Eye||Answers in Blood|
|Unforgiven||Blood Eagle||Boneless||The Choice||The Lord's Prayer|
|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|