An Expert 9×9 Puzzle Solved with Forced Chain in 25 Steps

This 9×9 board rates as expert (difficulty score 1269). Solving it from scratch takes 25 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, 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 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.
3. Then, 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.
4. After that, every remaining candidate in this region touches G6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
5. Now, that one can't be the marker: at F1 it would force G8, G9, H2, and 2 more cleared (together, two regions have open cells only in columns G and H), then E9 (the last open cell in region 9), and region 6 would be left with no open cell for its marker.
6. From there, marking G1 would force H3 (the last open cell in region 4), then F5 (the last open cell in region 5), then E9 (the last open cell in region 9), and then region 6 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 G5, G7, G8, and 1 more cleared (every open cell left in this region sits in column G, so its marker has to land there), then F5 (the last open cell in region 5), then G3 (the last open cell in region 4), and the chain continues, and region 6 would end up with no open cell for its marker, which can't happen. So it can't go there.
8. First, every remaining candidate in this region touches D2 and E2, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
9. Next, marking B2 would force C6, C7, D7, and 2 more cleared (every open cell in column A belongs to the same region, so that region's marker has to be somewhere in this column), then C5 (the last open cell in column C), then G4 (the last open cell in row 4), and then row 3 would be left with no open cell for its marker. So B2 can't be the marker there.
10. Then, if C2 were the marker, it would force E1 (the last open cell in region 2), then B7, D7, B8, and 1 more cleared (every open cell in column A belongs to the same region, so that region's marker has to be somewhere in this column), and column B would end up with no open cell for its marker, which can't happen. So it can't go there.
11. After that, if F2 were the marker, it would force D1 (the last open cell in region 2), then H8, H9, G8, and 1 more cleared (together, two regions have open cells only in columns H and G), then E9 (the last open cell in region 9), and region 6 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 G2, and region 4 would be left with no open cell for its marker.
13. From there, marking H2 would force G4 (the last open cell in region 4), and then region 5 would be left with no open cell for its marker. So H2 can't be the marker there.
14. Following that, place a marker at A2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column A, and its region.
15. First, every open cell in column B 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.
16. Next, every open cell in column C 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.
17. Then, place a marker at G4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column G, and its region. Placing here also clears its 2 touching neighbours.
18. After that, place a marker at C3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column C, and its region.
19. Now, place a marker at E5. Row 5 has exactly one open cell left. Placing here clears the rest of row 5, column E, and its region.
20. From there, place a marker at D1. Region has exactly one open cell left.
21. Following that, place a marker at F9. Column 6 has exactly one open cell left. Placing here clears the rest of row 9, column F, and its region.
22. First, every remaining candidate in this region touches I6 and I7, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
23. Next, 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. Placing here also clears its 1 touching neighbour.
24. Then, place a marker at H6. Region has exactly one open cell left.
25. Finally, place a marker at B7. Region has exactly one open cell left.

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