Halfdan was the brother of Norwegian King Harald "Finehair". Bjorn saved his life but he was later killed by his own brother.
He arrives in Kattegat in "Promised", along with the rest of his brother's ships and warriors: 20 ships carrying 600 warriors. When Harald introduced him to Ragnar, he seemed to not care about his presence. Later, his brother and him met Floki and Helga. Halfdan asked the boatbuilder why he has been in conflict with Ragnar despite the great role his longboat design played in Ragnar's achievements. Floki answered that it was because he killed his "pet christian" before they drank to their new friendship.
In "What Might Have Been", Halfdan ambushes the Frankish scouts alongside his brother, and is one of several Vikings seen torturing them by burning them alive. Halfdan even goes so far as to urinate on one of the scouts as he is being burned.
Halfdan is seen raiding a small Frankish farm in "Death All 'Round" with Harald and several other warriors. When the brothers discover the family living on the farm they rape and slaughter them, then burn it to the ground.
In "The Last Ship", Halfdan battles the Franks alongside Harald and the rest of their warriors. He slays several of them before being shot in the chest by a crossbow. Halfdan is later seen sailing back home with the other Vikings, injured, but still alive.
In the years following the defeat at the Second Siege of Paris, king Harald conquered the kingdom of Rogaland, killing king Arnarson and his earl Eyolf. He also defeated King Svein from an unspecified norwegian kingdom. Harald joins Bjorn in his expedition to the Mediterranean Sea and assures Bjorn he has no intention of conquering Kattegat. When Bjorn seeks a truce with his uncle Rollo, Harald is captured along with the other Norse leaders. He questions Bjorn's judgement in trusting Rollo. Eventually they are released when Bjorn agrees to allow Rollo to join the expedition.
Eventually, the fleet comes upon Algeciras and sacks the city. Floki prevents Harald and Halfdan from murdering praying muslims: Harald directly stops his brother from entering a conflict with Floki.
As the fleet returns to Scandinavia, Halfdan presents Harald to Egil. Harald recruits Egil as his agent and indirectly promises to make him earl. Egil begins to infiltrate Kattegat and locate weaknesess in it's defences. As the Lothbroks gather the Great Heathen Army, they ally with them once more. As recruits from all over Scandinavia arrives in Kattegat, Harald spots Ellisif. Halfdan is confused about Harald's nervous behavior upon seeing Ellisif and encourages him to talk to her.
Ellisif confesses to Harald that she is already married to a Danish earl, a man of lower social status and fame than Harald. Halfdan pulls a dagger to kill her. Ellisif begs him not to, and Harald lets her go. Later Halfdan suggests that they must kill Ellisif because she has taken Harald's honor. Harald refuses and reveals that he only loves two people: Halfdan and Ellisif. Harald states he will kill all those who threaten her.
Halfdan and Bjorn sail back to the Mediterranean Sea, making it to Sicily. He and Bjorn agree to served as bodyguards to its commander, Euphemius. However, Sinric discovers that Euphemius is merely a client of the Arab Emir Ziyadat Allah, and Bjorn asks to visit him. Euphemius refuses but is persuaded to agree by Kassia, a famous nun, and they set sail for Kairouan in Africa.
Once in Africa, Halfdan and Bjorn travel by caravan across the desert to meet Emir Ziyadat Allah, who welcomes them and offers to trade. Euphemius disappears and, at a banquet, the emir explains that he had defected to support the Emperor and has now been captured and executed. The Vikings realize that Euphemius is the meal they are eating and that Kassia is more powerful than she seems. Halfdan, Bjorn, and Sinric are seized and are about to be executed as a sandstorm approaches.
With the sandstorm as cover, Halfdan, Bjorn, and Sitric manage to escape, then sail back to Kattegat.
After his life was saved by Bjorn, Halfdan develops a friendship with him. After this, Harald decides to conquer Kattegat, and joins forces with Ivar the Boneless. Halfdan decides to join Lagertha, Bjorn and Ubbe.
Before the battle, the leaders of the armies decide on discuss terms in order to avoid needless slaughter. There, Harald attempts to persuade his brother to join him, though Halfdan refuses. Ivar claims that Halfdan cares too much on the fact that Bjorn saved his life. In return, Halfdan calls Ivar cynical, much to Ivar’s amusement.
At the first battle, Lagertha defeats Harald. Mid-Battle, the two brothers are seen glaring at each other. Having done this, Harald is then aided by Rollo who sends troops to aid him, though Halfdan and the forces/allies of Lagertha still agree to fight. During the battle, Halfdan fights bravely, though is confronted by his brother.
Harald wastes no time in impaling Halfdan, who has a hallucination of his time in the desert. Halfdan then dies, and is lamented by Harald.
Personality and traits
A wild and reckless warrior to whom combat seems almost a joyous pursuit, Halfdan is also shown to be incredibly cruel, as he takes great pleasure in torturing and killing his enemies. He seems to be the subservient one when compared to his brother Harald, typically speaking only after he does. He is also more unhinged than his brother.
|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|