An Expert 9×9 Puzzle Solved with Forced Chain in 28 Steps

This 9×9 board rates as expert (difficulty score 1506). Solving it from scratch takes 28 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Forced chain. Every step below is forced: nothing here is a guess.

1. First, every open cell in column F 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.
2. Next, marking A1 would force B8 (the last open cell in region 9), then C5 (the last open cell in region 4), and then region 8 would be left with no open cell for its marker. So A1 can't be the marker there.
3. Then, if B1 were the marker, it would force E4 (the last open cell in column E), and column D 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 E4 (the last open cell in column E), and column D would be left with no open cell for its marker.
5. Now, that one can't be the marker: at F1 it would force E4 (the last open cell in column E), and column D would be left with no open cell for its marker.
6. From there, marking G1 would force E4 (the last open cell in column E), and then column D would be left with no open cell for its marker. So G1 can't be the marker there.
7. Following that, if H1 were the marker, it would force E4 (the last open cell in column E), and column D would end up with no open cell for its marker, which can't happen. So it can't go there.
8. First, that one can't be the marker: at I1 it would force E4 (the last open cell in column E), and column D would be left with no open cell for its marker.
9. Next, 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.
10. Then, marking C3 would force I2 (the last open cell in region 3), then D1 (the last open cell in column D), and then column E would be left with no open cell for its marker. So C3 can't be the marker there.
11. After that, if G3 were the marker, row 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
12. Now, that one can't be the marker: place it at H3, and region 6 would be left with no open cell for its marker.
13. From there, 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.
14. Following that, marking A4 would force B8 (the last open cell in region 9), and then column G would be left with no open cell for its marker. So A4 can't be the marker there.
15. First, if B4 were the marker, it would force E1 (the last open cell in column E), then I2 (the last open cell in row 2), then F3 (the last open cell in row 3), and the chain continues, and column G would end up with no open cell for its marker, which can't happen. So it can't go there.
16. Next, marking F3 would force I2 (the last open cell in region 3), then A9 (the last open cell in row 9), then E1 (the last open cell in column E), and the chain continues, and then column D would be left with no open cell for its marker. So F3 can't be the marker there.
17. Then, that one can't be the marker: at C4 it would force I3 (the last open cell in row 3), then F2 (the last open cell in row 2), and column E would be left with no open cell for its marker.
18. After that, marking D4 would force E1 (the last open cell in region 1), then I2 (the last open cell in row 2), then B3 (the last open cell in row 3), and the chain continues, and then row 7 would be left with no open cell for its marker. So D4 can't be the marker there.
19. Now, if E1 were the marker, it would force I2 (the last open cell in row 2), then B3 (the last open cell in row 3), then A7 cleared (every open cell left in this region sits in column A, so its marker has to land there), and the chain continues, and row 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
20. From there, place a marker at D1. Region has exactly one open cell left. Placing here clears the rest of row 1, column D, and its region.
21. Following that, place a marker at E4. Column 5 has exactly one open cell left. Placing here clears the rest of row 4, column E, and its region. Placing here also clears its 1 touching neighbour.
22. First, place a marker at G8. Column 7 has exactly one open cell left. Placing here clears the rest of row 8, column G, and its region. Placing here also clears its 2 touching neighbours.
23. Next, place a marker at A9. Region has exactly one open cell left. Placing here clears the rest of row 9, column A, and its region.
24. Then, place a marker at B7. Row 7 has exactly one open cell left. Placing here clears the rest of row 7, column B, and its region.
25. After that, place a marker at C5. Region has exactly one open cell left. Placing here clears the rest of row 5, column C, and its region.
26. Now, place a marker at H6. Region has exactly one open cell left. Placing here clears the rest of row 6, column H, and its region.
27. From there, place a marker at F2. Region has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region.
28. Finally, place a marker at I3. Region has exactly one open cell left.

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