A Hard 8×8 Puzzle Solved with Forced Chain in 13 Steps

This 8×8 board rates as hard (difficulty score 947). Solving it from scratch takes 13 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Paired regions, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, every open cell left in this region sits in row 1, so its marker has to land there; that clears every other open cell in the row.
2. Next, every remaining candidate in this region touches G2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, marking G1 would force H7, H8, F3, and 3 more cleared (together, two regions have open cells only in columns H and F), then E7 (the last open cell in region 8), then B6 (the last open cell in region 7), and the chain continues, and then column F would be left with no open cell for its marker. So G1 can't be the marker there.
4. After that, if H1 were the marker, it would force G8, F2, F3, and 3 more cleared (together, two regions have open cells only in columns G and F), then E7 (the last open cell in region 8), then G6 (the last open cell in region 6), and column F would end up with no open cell for its marker, which can't happen. So it can't go there.
5. Now, place a marker at F1. Region has exactly one open cell left. Placing here clears the rest of row 1, column F, and its region. Placing here also clears its 1 touching neighbour.
6. From there, together, two regions have open cells only in columns H and G; with one marker each, those two markers have to fill exactly those two columns, so every other region's open cells there can be cleared.
7. Following that, place a marker at E7. Region has exactly one open cell left. Placing here clears the rest of row 7, column E, and its region. Placing here also clears its 2 touching neighbours.
8. First, place a marker at B6. Region has exactly one open cell left. Placing here clears the rest of row 6, column B, and its region. Placing here also clears its 2 touching neighbours.
9. Next, place a marker at H5. Region has exactly one open cell left. Placing here clears the rest of row 5, column H, and its region. Placing here also clears its 1 touching neighbour.
10. Then, place a marker at G3. Region has exactly one open cell left. Placing here clears the rest of row 3, column G, and its region.
11. After that, place a marker at C4. Region has exactly one open cell left. Placing here clears the rest of row 4, column C, and its region.
12. Now, place a marker at A8. Region has exactly one open cell left. Placing here clears the rest of row 8, column A, and its region.
13. Finally, place a marker at D2. Region has exactly one open cell left.

The key move here was Forced chain. Once you can spot that, this board falls quickly. ∎