The bishops return from vacation with a deity of their own to fight Cthulu.

A mysterious trapdoor opens in the floor next to the table…

The letters and numbers on the edge of the board are fixed so they accurately reflect the coordinate system of the expanded board size. (Except for the j-file, which remains unlabeled.)

/uj It’s a piece in 3d chess that can move along “space diagonals”, meaning it travels through the corners of a cubic space/square. Wiki link