A Hard 9×9 Puzzle Solved with Forced Chain in 22 Steps

This 9×9 board rates as hard (difficulty score 1037). Solving it from scratch takes 22 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, together, two regions have open cells only in rows 1 and 2; with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
2. Next, every remaining candidate in this region touches H4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, every open cell left in this region sits in row 5, so its marker has to land there; that clears every other open cell in the row.
4. After that, together, two regions have open cells only in rows 3 and 4; with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
5. Now, every remaining candidate in this region touches G4 and G6 and H6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
6. From there, every open cell left in this region sits in row 3, so its marker has to land there; that clears every other open cell in the row.
7. Following that, every remaining candidate in this region touches H2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
8. 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.
9. Next, if H1 were the marker, it would force G5 (the last open cell in region 6), then I3 (the last open cell in region 3), then F7 (the last open cell in region 5), and the chain continues, and region 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
10. Then, place a marker at I1. Region has exactly one open cell left. Placing here clears the rest of row 1, column I, and its region.
11. After that, together, two regions have open cells only in columns G and H; 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.
12. Now, every open cell left in this region sits in column F, so its marker has to land there; that clears every other open cell in the column.
13. From there, place a marker at E2. Region has exactly one open cell left. Placing here clears the rest of row 2, column E, and its region.
14. Following that, place a marker at D9. Region has exactly one open cell left. Placing here clears the rest of row 9, column D, and its region. Placing here also clears its 1 touching neighbour.
15. First, every open cell left in this region sits in row 7, so its marker has to land there; that clears every other open cell in the row.
16. Next, place a marker at F6. Region has exactly one open cell left. Placing here clears the rest of row 6, column F, and its region. Placing here also clears its 1 touching neighbour.
17. Then, 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.
18. After that, place a marker at G3. Region has exactly one open cell left.
19. Now, every remaining candidate in this region touches B8, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
20. From there, 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. Placing here also clears its 1 touching neighbour.
21. Following that, place a marker at C7. Region has exactly one open cell left. Placing here clears the rest of row 7, column C, and its region.
22. Finally, place a marker at B4. Region has exactly one open cell left.

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