Need some clarification about this song

I came across this in a private server game folder and it's been bugging me.

http://www.youtube.com/watch?v=JXCGJUVhDpA

The song is labeled as "Plague A.K.A Undead Prontera" (those of you who know from that right there probably see why I'm asking) and when I looked at the file info "梶浦由記" was tagged as the Album Artist and "You can do it!" as the Album.

Anyone know what it's actually called? I'm pretty sure that it's not a part of the OST because I've gotten it before and it wasn't there and the style is different from the rest of the songs.

Was this used in an area in game and I just didn't know? I have a feeling this is something different because there were other custom songs added in the BGM folder.