This puzzle is accessed by clicking the Moon in the background of The Seventh House menu screen. It is unlocked after opening The Third Gateway. Seven cups appear at the top of the screen, and fourteen cards appear below, which represent the curses and keys unlocked by completing the stained glass window puzzles.
Once all iterations of the stained glass window puzzles have been completed, you can click each curse card, then its corresponding key card, which will fill one of the cups at the top of the screen. When all the cups are filled, a large amount of jumbled text can be seen at the bottom of the screen.
Note: I am not sure what effect this puzzle has on the game, what the text means, or if it is a prerequisite to unlocking the Finale.