Secrets lurk everywhere in the Zombies mode of any given Call of Duty, and the recently-released WWII is no exception. If you've been screaming and struggling to survive The Final Reich, you may not have realised the prologue contains a secret room, or that there are six more playable characters to unlock.
A secret "mystery box" can be found behind a locked door on the tiny prologue map, Groesten Haus. This map is supposed to serve as a tutorial for the "real" game, but you can keep playing it as long as you stay alive. Ordinarily, you wouldn't be able to survive it for long, but the mystery box will help with that.
The box will offer a random weapon at the same 1000-Jolt price as the mystery box found in The Final Reich. And let's be honest, 1000 Jolts is a small price to pay for more firepower. Otherwise, you only have access to purchase the MP40 submachine gun, a self-revive perk and some armour, which sounds like a lot until you get 10 rounds into the prologue and burn through ammo too fast. The mystery box will be your friend.
The secret box is permanent, but the biggest issue is getting enough time to purchase a gun. I usually only have time to hit the box once, maybe twice, between rounds. The zombies spawn pretty quickly and don't understand the concept of personal space.
The mystery box will definitely be useful if you're trying to go high rounds before opening the door that prompts the end of the tutorial. Finding it is simply a matter of shooting 10 lanterns on the map to get the door to unlock on the upstairs level of the house. You'll hear a crow caw each time you hit one, and a tiny purple light will ignite on the inside of the lantern. Two of the lanterns are outside in the dark, and YouTuber Avery "Avxry" has a tutorial for finding them all.
However, you probably don't want to open the secret room if you're planning on unlocking all the secret characters. The Mountaineer has a brutal requirement of surviving 25 rounds in the Prologue without unlocking the secret room. This is a pretty difficult challenge, so it might take a few tries or maybe a few hundred without having the extra firepower of random mystery box guns.
Popular Zombies YouTuber "Chucky" has a video for unlocking the Mountaineer as well as five other secret soldiers: Hunter, Survivalist, BAT Agent, BAT Elite and the Assassin.
The new characters don't really seem to give any benefits, but they will probably be worth bragging rights, especially since the BAT Elite will require you to complete the Easter Egg all by yourself and also under time constraints.