The Second Battle for the Kingdom of Kattegat is a military engagement that is part of the Kattegat Civil War. It depicts the confrontation between Ivar, Hvitserk, and King Harald's forces against the forces of Lagertha, Björn, Ubbe, and Halfdan. The event takes place in Season 5 Episode 10, Moments of Vision.
The First Battle for the Kingdom of Kattegat had been a conclusive victory for Lagertha and Björn's forces, albeit not a decisive one. While the forces of Ivar and King Harald had sustained heavy losses and retreated back to the Kingdom of Vestfold-Rogaland, they received the help of Duke Rollo who arrives with his own forces. Duke Rollo brought approximately one thousand soldiers with him. Duke Rollo's forcers would ultimately turn the tide of the Kattegat Civil War in favor of Ivar and King Harald.
Before the battle, King Harald asks Queen Astrid to cut his hair, which he had promised he would not do until he married the woman of his dreams. He is blissfully unaware that Astrid has betrayed him and that the child she is carrying is possibly the son of one of the men who helped Astrid betray him and then gang-raped her. Harald notices that something is wrong and asks her to ease her heart. Astrid says she can't tell him, and Harald responds by kissing her.
Meanwhile, Björn asks Halfdan why chose to side with Björn over his brother. Halfdan replies that he did so because he did not want to fight his "Viking brothers" anymore and because Björn had saved his life in Ifriqiya.
The two armies meet facing each other in the open land of the Kingdom of Vestfold-Rogaland. Lagertha and Björn's army is stationed on a hilltop, while Harald and Ivar's forces wait to engage them at the bottom of the hill, positioned close to a small stream.
In the forest surrounding the battlefield, the Samis ambush a small part of Harald's forces, using stealth to take out several warriors. Unfortunately, the Samis are soon overwhelmed. Princess Snaefrid finds her father, King Svase, killed and starts to realize that the element of surprise they had in the previous battle is gone. The Samis are losing. Snaefrid takes up her father's weapons and begins to fight on, only to be cut down by a random warrior.
Simultaneously, Lagertha and Björn order their archers to open fire, and hundreds of arrow begin flying at the opposing army. Hvitserk and Harald are leading the front lines against Lagertha and Björn's army. Both sides charge across the field toward each other.
The battle very quickly becomes a furious and vicious close-quarters fight, with both sides trying to grain the upper hand. Lagertha fights with her usual strength and skills, cutting down all her face her. At one point she shows her absolute ruthlessness when she grabs a decapitated head and uses it as a mace in the battle. Hvitserk and Ubbe come face-to-face, but Ubbe finds himself unable to strike his brother down. In another scene of the battle, Ivar spots Lagertha from afar and imagines the whole battlefield as a field of fighting skeletons and Lagertha being the only one who is "real."
Astrid appears in front of Lagertha and asks her to kill her because her pregnancy might be proof of her betrayal. She tells her, "I can't have this child!" Lagertha's confused behavior upon learning that Astrid is pregnant only makes it more painful when Astrid runs into Lagertha's sword. She dies in Lagertha's arms who cries over the loss of her close friend and lover.
Elsewhere on the battlefield, Guthrum meets Hvitserk who kills him with ease, while Torvi is forced to watch her son's death from a distance, unable to help him. Hvitserk's gleefulness about it makes the whole thing close to nightmarish. Lagertha, still holding Astrid's body, witnesses all of this.
At the top of the hill, Ivar gives an impassioned and fierce war speech to inspire his troops, promising them entry into Valhalla. Ivar then launches his reserves into the field. Björn orders his archers to fire on Ivar. Ivar's bodyguards work to shield him and themselves, while Ivar stands calmly as if the rain of arrows is just regular rain.
Ivar and Hvitserk meet on the battlefield, and even though they are fighting on the same side, Ivar tells Hvitserk that he thinks Hvitserk regrets joining him. Then Ivar says that he does not think Hvitserk loves him, indirectly suggesting that he doesn't either. Hvitserk does not take this well.
Halfdan and Harald face each other down on the battlefield. Halfdan now suddenly finding himself in front of his brother. They pause and stare at each other for a moment. Then Harald cuts Halfdan down without a moment of hesitation. Halfdan seems genuinely taken by surprise about what has just happened. Then Harald realizes what he has done and seems unable to decide if he feels anger or sadness and starts to repeat "I will see you in Valhalla" over and over.
The army of Björn and Lagertha is barely holding their ground against the army of Ivar and Harald. Then Ivar sends in the Frankish forces sent by Rollo and completely dominates the battle. Björn's reaction of despair completely sells it as both an awe-inspiring and dreadful moment. Already pushed beyond their limits, Björn orders his side to retreat.
The battle is a victory and success for Ivar and King Harald. It ends the Kattegat Civil War with Björn, Lagertha, Ubbe, Torvi, Bishop Heahmund, and their remaining warriors being forced to abandon Kattegat to Ivar, and on the advice of Bishop Heahmund, travel back to Wessex. The battle took a heavy toll on both sides, but mostly on Lagertha and Björn's. It's also implied that Lagertha has finally developed PTSD when she has a flashback to her childhood in the middle of the battlefield, and is then missing afterwards. She is eventually found by Björn after she has suffered a complete mental breakdown inspired by Astrid and Guthrum's deaths. She also now has completely white hair.