Warhammer: Vermintide 2

Shortly after the outisde snowy section you'll find a dining room section where you must "unlock the gate" for the objective. There are levers throughout the room, you'll want to flip these in this order:

1-5-3-2

Once you do that, enter the secret room (go to the left of Lever 2) and flip the confirmation lever. This should unlock the achievement, and open up a secret path for one of the skulls if you are collecting those as well.