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

This 9×9 board rates as hard (difficulty score 1097). Solving it from scratch takes 22 logical steps, using 8 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line 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 column I, so its marker has to land there; that clears every other open cell in the column.
2. Next, 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.
3. Then, every open cell in row 1 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
4. After that, together, two regions have open cells only in rows 8 and 9; 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 B6 and C6 and B8 and C8, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
6. From there, place a marker at A8. Row 8 has exactly one open cell left. Placing here clears the rest of row 8, column A, and its region. Placing here also clears its 2 touching neighbours.
7. 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. Placing here also clears its 1 touching neighbour.
8. First, 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.
9. Next, place a marker at B5. Region has exactly one open cell left. Placing here clears the rest of row 5, column B, and its region.
10. Then, every open cell left in this region sits in row 6, so its marker has to land there; that clears every other open cell in the row.
11. After that, every remaining candidate in this region touches H3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
12. Now, every remaining candidate in this region touches G4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
13. From there, if E1 were the marker, it would force F6 (the last open cell in region 6), then I2 cleared (every open cell left in this region sits in row 2, so its marker has to land there), then H4 cleared (every remaining candidate in this region touches H4), and the chain continues, and row 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
14. Following that, 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.
15. First, that one can't be the marker: at F1 it would force E6 (the last open cell in region 6), then H2 (the last open cell in region 3), and row 3 would be left with no open cell for its marker.
16. Next, 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.
17. Then, if H1 were the marker, it would force G3 (the last open cell in region 5), and row 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
18. After that, place a marker at G1. Region has exactly one open cell left. Placing here clears the rest of row 1, column G, and its region. Placing here also clears its 1 touching neighbour.
19. Now, place a marker at H4. Region has exactly one open cell left. Placing here clears the rest of row 4, column H, and its region. Placing here also clears its 1 touching neighbour.
20. From there, place a marker at I2. Region has exactly one open cell left.
21. Following that, place a marker at F3. Region has exactly one open cell left. Placing here clears the rest of row 3, column F, and its region.
22. Finally, place a marker at E6. Region has exactly one open cell left.

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