"Vikings" Season 4 is slowly unfolding the tidbits for its next episode titled "The Great Army." Possible spoilers are indicated below for the continuity of the History Channel series. 

On season 4 episode 17, Ivar (Alex Hogh) continues on his plans to avenge the death of his parents, Ragnar (Travis Fimmel) and Aslaug (Alyssa Sutherland). He will seek the vengeance he craves after his father was thrown into a pit of snakes care of King Aelle (Ivan Kaye). According to the official synopsis on TV Guide, "A great Viking army assembles at Kattegat, while Lagertha (Katheryn Winnick) remains vigilant in the absence of Bjorn (Alexander Ludwig)."

Entertainment Weekly notes that the prior episode, titled "Crossings," showed how Lagertha chose to rule after the revelation of Ragnar's death. "Ragnar hated it... It weighed him down, perhaps it even killed him," she told her partner Astrid (Joesfin Asplund). However, later she realized that she must not disappoint those who fought and lost their lives to protect her.

Being the new Queen, her goal is to improve the lands of Kattlegat and create a true fortress out of the village. However, the sons of Aslaug, namely Ubbe (Jordan Patrick Smith) and Sigurd (David Linstron), are in doubt of her move, with Ivar apparently displeased.

Ivar said that he truly craves the justice for Aslaug, thus challenging Lagertha. The new queen declines. A threat against her is already foreseen, with Ivar saying that, one day, he will kill her and that there is no stopping it. Variety writes that it was revealed by producer and writer Michael Hirst in an interview that the attempts of revenge have already been planned and will come into fruition after Ragnar's passing.

"The Great Army" is slated to air on the History Channel on January 11, 2017. The next episode for "Vikings," titled "Revenge," is set for January 18.