An Expert 9×9 Puzzle Solved with Forced Chain in 36 Steps

This 9×9 board rates as expert (difficulty score 1573). Solving it from scratch takes 36 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 remaining candidate in this region touches C1, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
2. Next, every remaining candidate in this region touches D5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, every remaining candidate in this region touches F9, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
4. After that, marking A1 would force E2, F2, G2, and 2 more cleared (every open cell left in this region sits in row 2, so its marker has to land there), then D9, E9, G9, and 2 more cleared (every open cell left in this region sits in row 9, so its marker has to land there), then B8, C8, D8, and 3 more cleared (every open cell left in this region sits in row 8, so its marker has to land there), and the chain continues, and then column E would be left with no open cell for its marker. So A1 can't be the marker there.
5. Now, if E1 were the marker, it would force C2 (the last open cell in region 2), then D6 (the last open cell in region 6), then A7, B7, H7, and 1 more cleared (every open cell left in this region sits in row 7, so its marker has to land there), and the chain continues, and column I would end up with no open cell for its marker, which can't happen. So it can't go there.
6. From there, that one can't be the marker: at F1 it would force A2, B2 and H2 cleared (every open cell left in this region sits in row 2, so its marker has to land there), then G4, G5, G6, and 1 more cleared (every open cell left in this region sits in column G, so its marker has to land there), then E3, E4, I5, and 4 more cleared (every open cell in column H belongs to the same region, so that region's marker has to be somewhere in this column), and column I would be left with no open cell for its marker.
7. Following that, marking G1 would force F8 (the last open cell in region 9), then D9 (the last open cell in region 8), then C2 (the last open cell in region 2), and the chain continues, and then region 4 would be left with no open cell for its marker. So G1 can't be the marker there.
8. First, if H1 were the marker, it would force A2 and B2 cleared (every open cell left in this region sits in row 2, so its marker has to land there), then E3, F3, E4, and 5 more cleared (every open cell in column I belongs to the same region, so that region's marker has to be somewhere in this column), then B3 cleared (every remaining candidate in this region touches B3), and the chain continues, and row 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
9. Next, every open cell in column H 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.
10. Then, every open cell in column I 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.
11. After that, that one can't be the marker: at A2 it would force D1 (the last open cell in region 2), then E9, G9 and H9 cleared (every open cell left in this region sits in row 9, so its marker has to land there), then B8, C8, E8, and 1 more cleared (every open cell left in this region sits in row 8, so its marker has to land there), and the chain continues, and row 8 would be left with no open cell for its marker.
12. Now, marking B2 would force A4 (the last open cell in region 1), then D1 (the last open cell in region 2), then I3 (the last open cell in region 3), and the chain continues, and then row 8 would be left with no open cell for its marker. So B2 can't be the marker there.
13. From there, if C2 were the marker, it would force I1 (the last open cell in row 1), then A5, B5, B6, and 2 more cleared (together, two regions have open cells only in columns A and B), then D6, D8 and D9 cleared (every open cell left in this region sits in column D, so its marker has to land there), and the chain continues, and row 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
14. Following that, 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.
15. First, marking H2 would force D1 (the last open cell in region 2), then I4 (the last open cell in region 3), then A3 (the last open cell in region 1), and the chain continues, and then row 8 would be left with no open cell for its marker. So H2 can't be the marker there.
16. Next, if A3 were the marker, it would force E9, G9 and H9 cleared (every open cell left in this region sits in row 9, so its marker has to land there), then B8, C8, E8, and 1 more cleared (every open cell left in this region sits in row 8, so its marker has to land there), then C6 cleared (together, two regions have open cells only in columns C and B), and the chain continues, and row 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
17. Then, marking D1 would force I2 (the last open cell in row 2), then C4 and H4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), then A5, B5, C5, and 4 more cleared (every open cell in row 3 belongs to the same region, so that region's marker has to be somewhere in this row), and the chain continues, and then row 8 would be left with no open cell for its marker. So D1 can't be the marker there.
18. After that, place a marker at D2. Region has exactly one open cell left. Placing here clears the rest of row 2, column D, and its region. Placing here also clears its 1 touching neighbour.
19. Now, that one can't be the marker: place it at B3, and region 1 would be left with no open cell for its marker.
20. From there, marking A4 would force I1 (the last open cell in row 1), then H3 (the last open cell in row 3), then E9 and G9 cleared (every open cell left in this region sits in row 9, so its marker has to land there), and the chain continues, and then row 8 would be left with no open cell for its marker. So A4 can't be the marker there.
21. Following 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.
22. First, together, two regions have open cells only in columns C and A; 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.
23. Next, 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.
24. Then, every remaining candidate in this region touches F6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
25. After that, 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.
26. Now, every remaining candidate in this region touches B4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
27. From there, place a marker at B1. Region has exactly one open cell left. Placing here clears the rest of row 1, column B, and its region.
28. Following that, every remaining candidate in this region touches H3 and H4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
29. First, place a marker at I3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column I, and its region.
30. Next, place a marker at C4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column C, and its region.
31. Then, every remaining candidate in this region touches F8 and G8, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
32. After that, place a marker at G9. Region has exactly one open cell left. Placing here clears the rest of row 9, column G, and its region. Placing here also clears its 1 touching neighbour.
33. Now, place a marker at F7. Region has exactly one open cell left. Placing here also clears its 1 touching neighbour.
34. From there, place a marker at E5. Region has exactly one open cell left. Placing here clears the rest of row 5, column E, and its region.
35. Following that, 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.
36. Finally, place a marker at A8. Region has exactly one open cell left.

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