An Expert 9×9 Puzzle Solved with Forced Chain in 22 Steps

This 9×9 board rates as expert (difficulty score 1269). Solving it from scratch takes 22 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Line confinement, Paired regions, Forced chain. Every step below is forced: nothing here is a guess.

1. First, marking A1 would force I5, I6, H2, and 4 more cleared (together, two regions have open cells only in columns I and H), then G7 (the last open cell in region 7), then F5 (the last open cell in region 5), and then region 2 would be left with no open cell for its marker. So A1 can't be the marker there.
2. Next, every open cell in column A belongs to the same region, so that region's marker has to be somewhere in this column; that clears every other open cell in the region.
3. Then, if B1 were the marker, it would force I5, I6, H2, and 4 more cleared (together, two regions have open cells only in columns I and H), then G7 (the last open cell in region 7), then F5 (the last open cell in region 5), and region 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
4. After that, that one can't be the marker: at C1 it would force B9 (the last open cell in column B), then E5, E6, F6, and 5 more cleared (every open cell in column D belongs to the same region, so that region's marker has to be somewhere in this column), then E4 (the last open cell in column E), and column F would be left with no open cell for its marker.
5. Now, marking D1 would force B9 (the last open cell in column B), then E5, E6, F6, and 5 more cleared (every open cell in column C belongs to the same region, so that region's marker has to be somewhere in this column), then E4 (the last open cell in column E), and then column F would be left with no open cell for its marker. So D1 can't be the marker there.
6. From there, that one can't be the marker: at F1 it would force I5, I6, H2, and 4 more cleared (together, two regions have open cells only in columns I and H), then G7 (the last open cell in region 7), and region 5 would be left with no open cell for its marker.
7. Following that, marking G1 would force H4 cleared (together, two regions have open cells only in columns H and I), then F5 (the last open cell in region 5), and then region 2 would be left with no open cell for its marker. So G1 can't be the marker there.
8. First, together, two regions have open cells only in columns H and I; 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.
9. Next, place a marker at G7. Region has exactly one open cell left. Placing here clears the rest of row 7, column G, and its region. Placing here also clears its 3 touching neighbours.
10. Then, place a marker at F5. Region has exactly one open cell left. Placing here clears the rest of row 5, column F, and its region. Placing here also clears its 2 touching neighbours.
11. After that, place a marker at E1. Region has exactly one open cell left. Placing here clears the rest of row 1, column E, and its region. Placing here also clears its 1 touching neighbour.
12. Now, that one can't be the marker: at A2 it would force I4 (the last open cell in row 4), then H9 (the last open cell in region 9), and column B would be left with no open cell for its marker.
13. From there, if C2 were the marker, it would force D8 (the last open cell in region 6), then B9 (the last open cell in region 8), and region 9 would end up with no open cell for its marker, which can't happen. So it can't go there.
14. Following that, if I2 were the marker, it would force H9 (the last open cell in region 9), then A4 (the last open cell in row 4), and column B would end up with no open cell for its marker, which can't happen. So it can't go there.
15. First, place a marker at B2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column B, and its region. Placing here also clears its 1 touching neighbour.
16. Next, every open cell in row 3 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.
17. Then, place a marker at A4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column A, and its region.
18. After that, if D6 were the marker, column C would end up with no open cell for its marker, which can't happen. So it can't go there.
19. Now, place a marker at C6. Row 6 has exactly one open cell left. Placing here clears the rest of row 6, column C, and its region.
20. From there, 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.
21. Following that, place a marker at I8. Region has exactly one open cell left. Placing here clears the rest of row 8, column I, and its region.
22. Finally, place a marker at H3. Region has exactly one open cell left.

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