You are a miner who knows about the Monster Girl Encyclopedia. One day you're working in a salt mine when you hear panicked screams further down and spot your fellow miners fleeing from that direction. "It is the end of times! We're all gonna die!" one of them screams as they all run past you.
Curiousity overtakes you and you decide to see what has everyone in a panic. You head in the direction they were running from and come to a massive chamber where there is a strange glowing green symbol. In the center of that symbol are a confused Mindflayer and a Shoggoth.