45 groups of 45
This webgame is demonic. The only instruction is:
Make 45 groups of 45! (By combining two at a time.)
The categories are not provided to you - you have to figure them out during play. That's the most interesting part of this experience, I think. Late in my time playing this game, I realized that I had 50 or so category groups onscreen, and that several of the ones I'd identified actually needed to be combined into one much larger and more diverse category.
If you get dopamine from this kind of thing, it's going to ruin your day, so be careful!! I deliberately waited until I had some open time in my schedule.
The creator, Thomas Colthurst, created this puzzle to "commemorate 2025." You can see Thomas's other work here. I took one look at "the crossword puzzle that is also a sudoku" and immediately sent it to several people I know. Wild stuff!!