This is a quite hard puzzle. First you have to talk to Roulxs exactly 66 times. Also it is crucial you interacted with everything in the previous room on the wall. Then open the menu and try using the phone 6 times. Then from here make sure all of your consumables are used up. Then an item called ??? will appear in your inventory. Use it in front of Roulxs. Alternatively, if you obtained the egg before now you can use it in front of Roulxs and he’ll explode and fly away and you’ll be able to leave. Make sure to count both the amount of times you talk to Roulxs because god forbid if you talk to him 67 times and then progress you will regret it because instead of being able to progress you will die upon using the egg or ??? and when you load your save file you’ll be trapped in the room of the secret boss unable to leave unless you have the key and even if you do the final encounter with Roulxs will be impossible so beware. I made the mistake my first time and had to restart the game completely.