How do the thingy in the halls of arrogance that says ''When the light is gone. you may find the end of arrogance''?

At the VERY beginning of the Halls map, turn around and exit. This will take you to a room that splits left and right. You must go to both areas (after you go left, go to the right room, vise versa) and simply reach the exit of both areas (they both have 2 different areas each). After you clear both, finally enter the area that was behind you at the beginning again, clear that, and the upstairs area should be unlocked. The area leads to a lightning-based boss, so prepare for that too :P

(Read one of guides in the FAQ section for more info)