IMHO it is not very meaningful to separate tags like thor or valhalla. With named contents like these, you can easily just do a search for "thor"
or "valhalla"
and get effectively the same thing.
On the other hand, I think it would be useful to have tags for generic elements. For instance, heaven or concepts-of-heaven or afterlife for a question on Valhalla. If someone wants to look up questions on a heavenly afterlife location, manual searches are not likely to be very productive - there's too many such places to search, and there's you can't search for an afterlife location if you don't know its name.
Basically I think tags should be beneficial for searching related questions, without being redundant with the basic search function.