Call of Duty: Black Ops 4

The Forest is the area containing the Pack-a-Punch, it's behind the large mass in front of the mansion. You need to complete three mini-tasks to get tuning forks, which will unlock the forest.

To activate the tasks, you must find stones hidden in jars with smoke coming out of them. There's one in the Bedroom area, one in the Library/Study and one in the wine cellar. Once you find one, melee it to reveal the stone. Now get 4+ kills near it and you'll be able to hold interact to gaze into the stone. You will get one of three types of image appear, linked to one of three challenges:

Clock - You will be able to hold interact on one of three clocks in the mansion; Billiards Room, Main Hall or Dining Room. Once you do a circle will appear on the ground, spend time in this circle until it disappears and the tuning fork will appear by the clock.

Perk Machine - You must head to the shown perk machine (it’ll be in the Cellar, Mausoleum, Gardens or Terrace). You’ll then have to survive near the machine while some Nosferatus attack you, the last one will drop the tuning fork.

Painting/Other - You will have to find and interact with the painting/object shown to you. This will spawn a ghost, which you must escort to a certain place. When it reaches there, the tuning fork will appear.
 

• Sad Woman - Tea Room, next to wallbuy
• Scared Woman - East Gallery, next to hole in wall
• Woman with scratches - Music room, near holes in wall.
• Man and Woman - Smoking Room, next to Billiards Room entrance
• Doll - Entrance Hall, back left corner
• Noose - Wine Cellar, on a chair on the right side.
• Will - In a chest on the shelf to the left of exit to Cemetery.

Once you have all three tuning forks, you can interact with the large mass to open the Forest. As long as you focus on the tasks above and not waste too many kills before you activate the three stones, you should easily manage to do this before wave 7.