Who is telling sons of Ragnar about Ragnar's death?

ScreenerTV knows the answer:

But even with these events, there’s a single solitary question that has to be on the mind of every “Vikings” fan: Who is the one-eyed man cloaked in black (Andre Eriksen) that brought the news of his death to the sons of Ragnar Lothbrok? He appeared briefly in a previous episode, but was merely credited as “Black Cloaked Figure.”

Clearly though, he’s a higher power that is able to be in many places at once, coming to Ragnar’s sons when they need him most. So who is he? Luckily, creator Michael Hirst has shed a little light on that question.

During a Q&A on Facebook, Hirst revealed that the man in question is “Odin, the All-Father.” Typically depicted as only having one eye, Odin is the most prominent of the Norse gods worshiped by the Vikings.

MoviePilot also weighs in:

But the strange figure on the boat is no stranger at all. In fact, he made an appearance in the very first episode of Vikings — in the first scene, no less. So who is it?

In the first season of Vikings, Ragnar is surveying a battle scene littered with bodies when he spots a strange figure in the distance, selecting fallen warriors to ascend to valhalla.

This is Odin, a god who the vikings believed ruled valhalla and was one of the most iconic and revered figures in norse mythology. He was commonly associated with ravens, and gave his eye in exchange for knowledge.

So how do we know the man on the boat is Odin? Well, he's got one eye, and his ship is surrounded by a swarm of ravens. He also appears directly after Ragnar's death, linking him to the viking's tragic demise.