In Chapters 12 and 13 of Blair Witch you return to areas known from the previous chapters. Once again, you will find yourself in the camp. Proceed to the tree known from Chapter 4. The page below contains a walkthrough for both chapters.
You're back in the camp. Pick up the radio that will allow you to contact Carver. Look around the camp and take the document, figurine and a photo. Then go to the area where Bullet is sitting. There, you'll find a camera containing a new tape.
After playing back the recording, it turns out that the camera has a new feature - it allows you to see trails invisible to the human eye.
In Chapter 13, your task is to get to the tree you encountered in Chapter 4. During your journey, you need to rely on the camera. Watch the screen and look for white elements - they are fragments of the trail that will lead you to your goal. During the hike you also need to watch out for the red monsters. Touching them will end up with your death, so avoid them.
At some point, Carver will contact you. To get to the next location, you need to listen to his instructions on where you should turn. This way, you will reach a path that will lead you safely to the tree. When you arrive, all is left to do is to break the branches.
Then you need to go further. Follow the trail until you reach its end. Now, you need to follow Bullet.
While following in his footsteps, you will encounter a dead deer. Further along the way, you will also encounter a small cave. Order Bullet to enter it. When he runs back out, pat him and continue forward. At some point, you will find a weapon. The chapter ends in that moment.