On the bookshelf located in the Fortress of Doom, there is a book titled "How to Comb your Mustache" by Cliffton M. Fischbach, the father of YouTuber Markiplier, who passed away due to lung cancer. The developers included his name as an easter egg in the game because Mark Fischbasch stated that his father got him into the very first game in the series.
Contributed by Tuli0hWut