It may be safely concluded that the eponymous Theater of Mystery in Blade & Soul’s update of the same name does not refer to new dungeons. Not just because the two new dungeons are named Dreamsong Theater and Den of the Ancients, but because we now know so much about these instances that they could not be construed as mysterious. They even include the new Dungeon Challenge mode, and you’ll need to know the dungeons really well for that, as players are completing for leaderboard slots based on the fastest clears.
There are also new events arriving, with some creepy tin man costumes in Hongsil’s Secret Storehouse. You can also access the Tower of Memory event dungeon until January 16th, take part in the Royal Raid event, hope for luck in the Joy to the Realm Jackpot event… in other words, there are plenty of fun seasonal events because of whatever holiday takes place in December. Check out the full rundown for shop prices on the official site, and read a little more about the two new dungeons in the patch on December 5th.
Source: Official Site