Onice you have all 3 ???, explore the final area a few more times and it should appear. (This might have been unclear ingame, sorry.)
I spammed the explore button in the final area, but I still couldn't get it. tbh, I had to look at the code to figure out the ???'s. I saw the bookpools and saw the "tendency" requirements and linked that with the potion lores to get the ???'s books. I can't find '3', the final message, anywhere in the bookpools, so I wonder if that's the problem.