Call of Duty WW2 Zombies - How to Unlock Secret Mystery Box Easter Egg in Prologue / Gresten Haus. This is very useful for the Pressure Cooker trophy & achievement (beat round 20 in Prologue).

There are 10 lanterns on this map that you must shoot. 8 are inside the house (4 downstairs, 4 upstairs) and 2 are outside (left window, right window). When shooting the lanterns they make a raven sound and will start glowing purple.
The video shows all 10 lantern locations.

After shooting all 10 lanterns a door opens upstairs, revealing the secret Mystery Box behind it. It costs 1000 points to use and gives you one random weapon. It can also give you Jack-in-the-Box which distracts zombies for a few seconds and kills them upon detonation.