Which episode in the Viking TV series is considered the most pivotal turning point in the show's narrative?

The most pivotal turning point in the narrative of the Vikings TV series is generally considered to be the episode titled 'All His Angels', which is the 15th episode of the fourth season. In this episode, a major character is killed off, which has a profound impact on the remaining characters and the overall story.

'All His Angels' is also significant in that it marks a transition from one major storyline to another, as the show moves away from the events in Paris and shifts its focus back to Scandinavia. This episode is regarded by many fans and critics as one of the most memorable and emotionally impactful moments of the entire series.