Just showing off the Wunderwaffe for the new zombie map "Call of the Dead." You can get it by doing an elaborate easter egg(although this one isn't as bad as Ascension was, at least solo) which involves freeing the old zombie team from behind the door.
If you're interested in finding out how to get it, there are already vids on how to do it; search "stand in call of the dead" or something similar to that.
Notes about the Wunderwaffe:
-Limited ammo. It seems to act like the Thunder gun in that it can't make the zombies drop any powerups, so aside from knifing, you can't really get a max ammo to test out if that would refill it(easter egg has not been done on co-op yet).
-You cannot Pack-A-Punch the weapon. Similar to the death machine, you can't buy anything off the wall, use the mystery box, use the Pack-A-Punch, or switch weapons while it is out.
-Once you use up all the ammo, you revert back to your regular guns
-As shown at the end of the video, apparently you can time-out of its use, although you can still hold it a *really* long time.
Notes on the video:
-Just showing where you first get it, and what it does and such
-When killing Romero, instead of dropping a death machine, he drops another Wunderwaffe, with the same timer as your previous Wunderwaffe (aka until you run out of ammo). Awesome!
-It's not shown, but regular death machine drops are still just regular death machines. Only the Romero drop is changed to the Wunderwaffe.