So I just noticed that a new achievement popped up in the list. As the Mark mask one, it's still a secret, but it involves doing (or something) ten times
Link to screencap: http://puu.sh/4XlZb/7ffcfbb2df.png
Edit: "?" has been split up into ten different achievements with rather cryptic descriptions, but that's what Overkill likes to do for events
Edit 2 (29th. October): The "progression" achievements had their values changed. I think it's suspicious that two achievements had their value changed to "666". Particularly easy tasks, or a hint?
The achievements were updated 30 minutes, the testing depot was updated 3 times today, and the last update was 1 hour ago.
Edit 3: Halloween Update revealed. It follows the Birthday Event's three day unlock system, meaning we get the revealed stuff at Thursday. Check in every day to see what we'll get at http://www.overkillsoftware.com/halloween/
Just found out how to activate the music click the pumpkin face! Also click the tip of the shotgun to take you to their twitter page? Also the new gun slide activates the music
Edit 4 (30th. October): Four more achievements without descriptions have been added, going "First Nightmare" to "Fourth Nightmare". Furthermore, the second reward has been revealed to be four masks and six patterns
Edit 5 (31st. October): Halloween Update is live now with the last day revealing the mask inventory increase and the achievementshttp://www.overkillsoftware.com/halloween/index.html