I've seen people crossing Neon Genesis Evangelion with My Little Pony a lot, and because I do a lot of roleplaying I decided to cross them too. So when it came to the topic of the angel Bardiel, I thought "This angel takes things over and ends in them being brutally slaughtered." So the original choice was to have it possess Fluttershy for maximum emotional impact. However she has no physical strength--something Bardiel was good at. So for that reason I decided to have our Thirteenth Angel (or in Rebuild, it is the ninth angel) take Applejack. While I myself find no emotional impact on sacrificing her, she does fit the bill in terms of strength.

The song is from Evangelion 2.22, "Les Betes" and is a remake of "The Beast" from the original series. Fun fact, the chorus at the start of the song is actually the Hallelujah song.