An out of bounds clip allowing you to reach any locked area. In the video I go straight to the last level - Catacombs. However due to the progress being locked behind completing missions it doesn't seem like one is able to actually complete the game by using this.
The clip itself is pretty difficult, requiring fairly precise positioning and a sprint to actually go through the wall.
![](https://i.ytimg.com/vi/mRiR305FJCI/maxresdefault.jpg)