Mysterium is a small but long mineshaft, that largely comprises metal parts, hard jumps, and purple cobblestone blocks are the reasons that the walls are purple that give it its mysterious look, hence its name "Mysterium". Mysterium is a highly decorated map. So highly decorated, that it causes lag for many players. There are lots of hard jumps, and similar to a mine, they are all in small spaces. Third person and first person view are hard to work with, causing the death of slow players. When this was added into the game, Crazyblox added a purple tint to things to make it more mysterious.
Because the map is long and contained within a small area, third person view may be a problem. The player can use first person view to make the map a little easier.
This map can be found in FE2 Map Test under the name "Enigmatic Mines". Enigmatic Mines has the exact same gameplay as this map, but has a different theme.