'Vikings' season 4 episode 16 spoilers: Ivar and Lagertha clash

Ragnar Lothbrok (Travis Fimmel) may be in Valhalla feasting with Odin right now, but the situation in Kattegat is about to get a little heated up once Ivar the Boneless (Alex Høgh Andersen) finds out that Lagertha (Katheryn Winnick) killed his mother Aslaug (Alyssa Sutherland).

As seen in the trailer, Ivar will challenge Lagertha to a single combat in "Vikings" season 4 episode 16. Lagertha knew this was coming, and Aslaug even warned her that her sons would avenge her. In an interview with The Hollywood Reporter last month, showrunner Michael Hirst said that "Lagertha wouldn't be Lagertha unless she did that."

"And by doing it of course she challenges the gods because she knows that revenge is basic to Viking society, and the sons of Aslaug will be damned to try and avenge their mother. So she makes this huge decision, but it's a decision to kill that was based on her love for Ragnar," he added.

Now, she has to deal with Ivar. Ivar is one of the most ruthless Vikings in history, and a fight with him probably will not end well for Lagertha. Ivar just lost a father in England, and now he finds out that his mother is also dead. This is the one son of Ragnar who is definitely thinking about vengeance.

Meanwhile, Bjorn (Alexander Ludwig) and his band of raiders will attack a settlement. During the battle, Floki (Gustaf Skarsgård) tells King Harald (Peter Franzén) and Halfdan (Jasper Pääkkönen) that if they want to kill the inhabitants, they will have to kill him first. Has Floki developed a soft spot for the Christians?

It seems Harald (Peter Franzén) is also planning to get rid of Bjorn because Ragnar's son may be a threat to his ambition of becoming the King of Norway.

"Vikings" season 4 episode 16 will air on Wednesday, Jan. 4, at 9 p.m. ET on History.