Revenge seems to be the theme for Vikings season 4 as every lead character is avenging the loss they suffered. King Ragnar Lothbrok, who was reeling from the death of his friend Athelstan, punished Floki by torturing him in a cave. Lagertha reclaimed the Earldom after killing Kalf right before their wedding. But the show's high point will occur in episode 6 titled What Might Have Been as it will feature the biggest battle between the Viking brothers, Ragnar and Rollo.

The synopsis of the episode reveals that the king of Kattegat will attack Paris again and this time to settle scores with his little brother who has turned rogue. In the previous episode, Rollo and his French army killed the remaining Vikings on the land of Paris.

The official synopsis of episode 6 reads: Ragnar declares another raid on Paris at the Thing in Kattegat; Ecbert also has a journey in mind as he dispatches Aethelwulf and Alfred on a pilgrimage to Rome.

Click here to watch the episode via live stream online on the History Channel.

The episode will focus on the rough relationship between the brothers. The Norse king's attack on Paris will be answered by Rollo's loyalty to his newlywed wife Gisla, the daughter of Emperor Charles. "I'll not betray you, I'll not betray Paris," Rollo promises King Charles in the episode 6 promo.

Rollo knows how difficult it is to defeat his brother and alerts Charles. "As long as my brother is still alive, he is not defeated," he warns.

However, the emperor of Paris needs to be cautious of the younger brother of the Norse ruler as he has never proved his loyalty to anyone. He has betrayed his own kin twice already. And this time, the Vikings want their leader Ragnar to finally avenge the death of their fellow men who were backstabbed and killed by Rollo in the premier episode.

"When everyone wanted you dead, I kept you alive," Ragnar tells his brother during their face-off and adds, "One of us will die today."

But Rollo, who is now a powerful leader after marrying Gisla, tells his elder brother, "And it won't be me."

Vikings season 4 airs every Thursday on History Channel.