So, how does one discover this hidden location? Redditor Cozmosus found this when he played in Practice Mode. He then went to the Deck Selection screen and pressed the asterisk key on the numpad, giving a hidden Naxxramas image that seems to have been left during the mode's development. Additionally, someone in the thread has identified that it is a simplified map of the Naxxramas raid in World of Warcraft.
The map looks like this:
<image source> <source>
