Vikings season 4 spoilers: Bjorn's life in danger from Legartha's partner Kalf?

After featuring a fierce bear fight in episode 3, Vikings season 4 will return on History Channel next Thursday (10 March) with another exciting chapter. King Ragnar Lothbrok is sinking into deeper problems after his friend Athelstan's death. His son Bjorn captured Floki for murdering the Christian monk and now the disgraced shipbuilder is confined to a cave prison awaiting his apparent death.

The upcoming episode is titled Yol and will reveal Viking warrior Kalf, who shares power with Legartha, trying to finish off Ragnar's son after Erlender informs him that Bjorn in living out in the Norwegian woods in the snowy winter. But what the attacker does not know is that Ragnar's blood is enduring the harsh conditions to sharpen his survival and war skills and that he has already won a fight with a grizzly bear.

A short teaser released by History Channel shows few clips of a huge sword fight inside the snow-filled woods and it will be interesting to see whether Kalf succeeds in his evil intentions by killing Ragnar's favourite son.

Another short clip of the same episode reveals that Floki might be able to escape the water-dripping cave. In episode 3 that aired on 3 March, a mysterious man had sent a mercy plea for Floki to the Norse ruler.

The official synopsis of the next episode reveals that Rollo's betrayal of his Viking lads is paying back. After his wife Princess Gisla initiated a divorce, his position as the future ruler of Paris is threatened.

The synopsis reads: King Aelle goes to Wessex for Christmas and is not impressed with how family relations are developing; Rollo's future hangs in the balance.

Vikings season 4 airs every Thursday night on History Channel.