I got all the stones (including elusive zero) and am now curious, what’s the third difficulty option that gets blanked out? And what’s the meaning of “will you discover the █̸̟̘̜̑̏u̴͍̐̊█̸͕̆l̸̡̝͈͗s̵̩̩͊̇̈█̷͍̰̇͛█̷̧̰́ņ̶̣͇͐̃͠█̵̺͕͊̅͑ ?” I tried replacing those blanks (?u?ls??n?), but doesn’t seem there’s any word matching that pattern. I feel like there’s still some super duper secret I’m missing… I did notice that my winning deck against the first tutorial dude is supposedly this:

But there’s no way that I had 59-stone in my deck when I beat them and actually the icon shown there isn’t the same as 59-stone so idk. I don’t recognise ever using a deck like that, and I can’t replicate it without knowing what that mysterious ghost + black stone is. Could I get a pointer please?

Nope, you've found everything. 0-stone = "nullstone", but I guess I didn't actually put that in the game's writing anywhere! Oops.

The third difficulty is something I plan to add in the near future, not in the game yet.

As for the winning deck against battle 1, it might be because you lost the initial tutorial fight, and then came back and beat it much later? Or it's a glitch. I have absolutely no idea, I guess a random unused sprite somehow got loaded in-place of X-stone or something??? The Unity Resources folder is weird sometimes I guess.

Eureka -- I figured out the weird sprite! Even though it's an old version of 59, it's also used in the game as 65-stone. You used 65- and 5-stone in your deck to solve a puzzle, then went back to beat the tutorial. No glitch involved!

[I somehow missed this when looking at the spritesheet, I only noticed when watching the posted video where the player accidentally made 65 while deckbuilding!]