Thanks, I can't think of a way to hint further without straight up telling so I'll just tell ya haha hope you don't mind.
SPOILERS:
The one you're missing is the true ritual ending, where you also use your blood to draw the pentagram in addition to doing the main ritual. The hint is a little cheeky. In document 2, it says you must find the vision of the creator. If you look closely at the picture beside the drawer, you'll see it is a highly distorted face. It's the face of the game's programmer, WinTH03 LMAO. Interact with the picture after you've done the main ritual, and you'll be able to obtain a knife to use at the board.
If I misunderstood and you've already got the blood ritual ending, simply enter the red door without using the knife.