Valhalla vs the empyrean

Valhalla

name
  • The home of half of all warriors who died gloriously in battle as well as many of the Æsir. 

noun
  • An abode of the gods or afterlife in general. 

the empyrean

How often have the words Valhalla and the empyrean occurred in a corpus of books? (source: Google Ngram Viewer )