No it's not on-topic, but aspects of the show may be.
The show Vikings is marketed on Wikipedia as a historical drama not a fantasy. It seems to be quite focused on both historical accuracy and portraying an accurate representation of Vikings. So no, it's not a work of fantasy and is therefore not on-topic.
However, as described in this meta on non-SF properties with occasional SF-nal elements that does not making everything about the show off-topic.
If there are particularly fictional elements of the show that one wants to ask about, they are welcome to. I will however note that prophets and gods are not necessarily fantastical, they very much exist in our world and are believed to be real by a large portion of our populace. Similar to being able to interact with the dead, although they are usually considered fantastical, as are ghosts/spectres (as mention, in passing, here).
There may be other such elements that are fantastical that one can ask about, but in general no, the show is not on-topic and questions should be redirected to our sister site M&TV.