I’ve identified Vanitas and Xehanort(s) and Young Xehanort on the board, but I’m not sure about anyone else.

 

Is there a dedicated breakdown anywhere/anyone has?

 

I’m also still baffled by the chalkboard drawing in Yen Sid’s tower corresponding to one of the new 7 Darknesses...

 

 

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This scan from the KH Ultimania shows an artwork that apparently has all the pieces we see in the chess game in different scenes.   
https://pbs.twimg.com/media/D0l2LBwV4AEFqFq.jpg

On 6/26/2019 at 12:25 PM, Sinre said:

I’m also still baffled by the chalkboard drawing in Yen Sid’s tower corresponding to one of the new 7 Darknesses...

I assume you're talking about the emblem of the Book of Prophecies. As far as we know, it represents the Master of Masters. At the moment, nobody knows why the emblem was on Yen Sid's chalkboard.

All I can say is that it doesn't seem unreasonable for it to be there/for him to know about it, seeing as how he's a keyblade master himself. If Brain was the one who Luxu bestowed the No Name to, then that means that Brain may have passed his knowledge of the Book down to future keyblade masters, including Yen Sid. That's just one possibility.