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Yes nice! I suggest to add a definition for "cardinal". Is that synonym of Orthogonal in this context? Maybe some diagrams would help understanding the moves. I have seen a typo : heaens. Why these name of Imp and Fiend? I have to check, I don t know these words in English.

I've fixed the file names, alongside a few formatting bugs and making sure pawns don't share a letter with any other piece.

While I understand why Earth got the a-h files, I would suggest giving them to the skies, while Earth gets the middle ones (j-q). This makes the files be in alphabetical order when the board is setup, and as an added nice bonus, the "missing" files i and r keep the color alternation. vg a1 white, b1 black, ..., h1 black, (i1 white), j1 black, ..., q1 white, (r1 black), s1 white, ..., y1 white, z1 black.

Nice!

It's particularly interesting how some of the 1-piece endgames (rook, angel) end up looking like a classical one to force the king out of Earth and then having the friendly king deliver the final attack. (The angel's isn't completely clear to me, since the lone king may move to the underworld early to avoid the angel's attacks, but I guess that's a significant enough detriment that then the friendly king can manage on his own.)