Bendy and the Ink Machine

You can get this in Chapter 3. During one point in the story, you will be asked to perform a few challenges. During the last challenge (Collect 5 ink hearts) you will be told to go to level 14. Take the elevator down to floor 14 and proceed down the stairs until you are in the ink. Be careful as there will be an enemy walking around when you come down the stairs. Let him pass.

Before continuing either left of right (staring at the Bendy statue) you will want to go around the back of the stairs you came down. You will need to break a few wooden boards and eventually you will get to a lone valve. Turn it. Once turned, go back up to Floor 9.

Once you are on Floor 9, go into the right door (next to the save machine). Climb the stairs until you get to a floor with 3 dressers and 2 cogs on the floor. From here you should see a now open path forward. Go straight into the open path, down the stairs and you will see the diary in front of you.