An Expert 9×9 Puzzle Solved with Forced Chain in 27 Steps

This 9×9 board rates as expert (difficulty score 1320). Solving it from scratch takes 27 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 column A, so its marker has to land there; that clears every other open cell in the column.
2. Next, every remaining candidate in this region touches C2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, that one can't be the marker: at C1 it would force E2 (the last open cell in region 2), then D4 cleared (together, two regions have open cells only in columns B and D), then G3 cleared (every remaining candidate in this region touches G3), and the chain continues, and column I would be left with no open cell for its marker.
4. After that, every open cell left in this region sits in column B, so its marker has to land there; that clears every other open cell in the column.
5. Now, every remaining candidate in this region touches C8, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
6. From there, marking D1 would force B2 (the last open cell in region 1), then C9 (the last open cell in region 9), then E3, E4, E5, and 1 more cleared (every open cell left in this region sits in column E, so its marker has to land there), and the chain continues, and then row 4 would be left with no open cell for its marker. So D1 can't be the marker there.
7. Following that, if E1 were the marker, it would force B2 (the last open cell in region 1), then D3 and D4 cleared (together, two regions have open cells only in columns C and D), then G3 cleared (every remaining candidate in this region touches G3), and the chain continues, and column F 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 D3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
9. Next, that one can't be the marker: at F1 it would force B2 (the last open cell in region 1), and region 2 would be left with no open cell for its marker.
10. Then, marking G1 would force B2 (the last open cell in region 1), and then region 2 would be left with no open cell for its marker. So G1 can't be the marker there.
11. After that, if H1 were the marker, it would force B2 (the last open cell in region 1), and region 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: at I1 it would force B2 (the last open cell in region 1), and region 2 would be left with no open cell for its marker.
13. From there, place a marker at B1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column B, and its region.
14. Following that, that one can't be the marker: at D2 it would force C9 (the last open cell in region 9), then E4, E5 and E6 cleared (every open cell left in this region sits in column E, so its marker has to land there), then G3 cleared (every remaining candidate in this region touches G3), and the chain continues, and row 4 would be left with no open cell for its marker.
15. First, marking E2 would force D4 cleared (together, two regions have open cells only in columns C and D), then G3 cleared (every remaining candidate in this region touches G3), then F5 (the last open cell in region 5), and the chain continues, and then column I would be left with no open cell for its marker. So E2 can't be the marker there.
16. Next, place a marker at C3. Region has exactly one open cell left. Placing here clears the rest of row 3, column C, and its region. Placing here also clears its 1 touching neighbour.
17. Then, every open cell left in this region sits in column D, so its marker has to land there; that clears every other open cell in the column.
18. After that, every open cell left in this region sits in column E, so its marker has to land there; that clears every other open cell in the column.
19. Now, 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.
20. From there, every open cell in row 2 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.
21. Following that, place a marker at I4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column I, and its region.
22. First, place a marker at A6. Row 6 has exactly one open cell left. Placing here clears the rest of row 6, column A, and its region.
23. Next, every remaining candidate in this region touches D8, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
24. Then, 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.
25. After that, place a marker at E7. Region has exactly one open cell left. Placing here clears the rest of row 7, column E, and its region.
26. Now, place a marker at G8. Region has exactly one open cell left. Placing here clears the rest of row 8, column G, and its region.
27. Finally, place a marker at H2. Region has exactly one open cell left.

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