Play the SUDOCUBE

Every clue lies on a single diagonal: the cell where slice X = k, Y = k and Z = k all meet holds the digit k. Nine digits, nine cells, one straight line through the body of the cube — and because each cell is seen from three directions, all 27 slices start with exactly one clue: slice k is empty but for a k.

There is a cube behind those nine clues, and reveal will give you its digits one at a time. But nine clues cannot pin down 729 cells, so you will not deduce your way to it: build a cube that keeps every slice legal, fill all 27, and however you got there you have finished it.