An Expert 10×10 Puzzle Solved with Forced Chain in 40 Steps

This 10×10 board rates as expert (difficulty score 1828). Solving it from scratch takes 40 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 B9, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
2. Next, marking A1 would force I2 (the last open cell in region 4), then C3 (the last open cell in region 2), and then region 3 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 C3 (the last open cell in region 2), then I2 (the last open cell in region 4), and region 3 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 I2 (the last open cell in region 4), then A3, B3, E3, and 1 more cleared (every open cell left in this region sits in row 3, so its marker has to land there), then A9, A10 and B10 cleared (together, two regions have open cells only in columns A and B), and region 10 would be left with no open cell for its marker.
5. Now, if E1 were the marker, it would force I2 (the last open cell in region 4), then C3 (the last open cell in region 2), then A9, A10 and B10 cleared (together, two regions have open cells only in columns A and B), and region 10 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 I2 (the last open cell in region 4), then C3 (the last open cell in region 2), then A9, A10 and B10 cleared (together, two regions have open cells only in columns A and B), and region 10 would be left with no open cell for its marker.
7. Following that, that one can't be the marker: at A2 it would force D1 cleared (every open cell left in this region sits in row 1, so its marker has to land there), then C3 (the last open cell in region 2), and region 3 would be left with no open cell for its marker.
8. First, marking B2 would force E3, G3, H3, and 2 more cleared (every open cell left in this region sits in row 3, so its marker has to land there), then A9, A10, C4, and 3 more cleared (together, two regions have open cells only in columns A and C), and then region 10 would be left with no open cell for its marker. So B2 can't be the marker there.
9. Next, every remaining candidate in this region touches D2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
10. Then, marking G1 would force C4, C5, C6, and 4 more cleared (every open cell left in this region sits in column C, so its marker has to land there), then J2, A3, B3, and 4 more cleared (together, two regions have open cells only in rows 2 and 3), then A9, A10 and B10 cleared (together, two regions have open cells only in columns A and B), and then region 10 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 C4, C5, C6, and 4 more cleared (every open cell left in this region sits in column C, so its marker has to land there), then J2, A3, B3, and 4 more cleared (together, two regions have open cells only in rows 2 and 3), then A9, A10 and B10 cleared (together, two regions have open cells only in columns A and B), and region 10 would end up with no open cell for its marker, which can't happen. So it can't go there.
12. Now, every remaining candidate in this region touches J2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
13. From there, that one can't be the marker: at I1 it would force C4, C5, C6, and 4 more cleared (every open cell left in this region sits in column C, so its marker has to land there), then A3, B3, E3, and 3 more cleared (together, two regions have open cells only in rows 2 and 3), then A9, A10 and B10 cleared (together, two regions have open cells only in columns A and B), and region 10 would be left with no open cell for its marker.
14. Following that, marking J1 would force C4, C5, C6, and 4 more cleared (every open cell left in this region sits in column C, so its marker has to land there), then H2, A3, B3, and 4 more cleared (together, two regions have open cells only in rows 2 and 3), then A9, A10 and B10 cleared (together, two regions have open cells only in columns A and B), and then region 10 would be left with no open cell for its marker. So J1 can't be the marker there.
15. First, place a marker at I2. Region has exactly one open cell left. Placing here clears the rest of row 2, column I, and its region. Placing here also clears its 2 touching neighbours.
16. Next, place a marker at D1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column D, and its region.
17. Then, place a marker at F3. Region has exactly one open cell left. Placing here clears the rest of row 3, column F, and its region. Placing here also clears its 2 touching neighbours.
18. After that, marking A4 would force E10, G10, H10, and 1 more cleared (every open cell left in this region sits in row 10, so its marker has to land there), then H5, H6 and H7 cleared (every open cell left in this region sits in column H, so its marker has to land there), then G6 cleared (every open cell in column J belongs to the same region, so that region's marker has to be somewhere in this column), and the chain continues, and then row 10 would be left with no open cell for its marker. So A4 can't be the marker there.
19. Now, that one can't be the marker: at C4 it would force G8 and G9 cleared (every open cell in column E belongs to the same region, so that region's marker has to be somewhere in this column), then A9, A10 and B10 cleared (together, two regions have open cells only in columns A and B), and region 10 would be left with no open cell for its marker.
20. From there, if H4 were the marker, it would force J10 (the last open cell in column J), then A9 (the last open cell in region 10), then C8 (the last open cell in region 8), and region 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
21. Following that, that one can't be the marker: at A5 it would force J4 (the last open cell in row 4), then E10, G10 and H10 cleared (every open cell left in this region sits in row 10, so its marker has to land there), then H7 cleared (every open cell left in this region sits in column H, so its marker has to land there), and the chain continues, and column E would be left with no open cell for its marker.
22. First, marking B5 would force J4 (the last open cell in row 4), then E6 (the last open cell in row 6), then G10 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 the chain continues, and then column G would be left with no open cell for its marker. So B5 can't be the marker there.
23. Next, if C5 were the marker, it would force J4 (the last open cell in row 4), then E6 (the last open cell in row 6), then A7 (the last open cell in row 7), 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.
24. Then, marking H5 would force J10 cleared (every open cell left in this region sits in column J, so its marker has to land there), then G10 (the last open cell in region 9), then A9 (the last open cell in region 10), and then column E would be left with no open cell for its marker. So H5 can't be the marker there.
25. After that, that one can't be the marker: at J5 it would force B4 (the last open cell in row 4), then C6 and C7 cleared (together, two regions have open cells only in columns A and C), then E6 (the last open cell in row 6), and the chain continues, and column G would be left with no open cell for its marker.
26. Now, every open cell in row 5 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.
27. From there, if A6 were the marker, it would force J4 (the last open cell in row 4), then C7 (the last open cell in row 7), and region 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
28. 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.
29. First, together, two regions have open cells only in columns A and C; 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.
30. Next, together, two regions have open cells only in columns E and G; 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.
31. Then, marking J4 would force B6 (the last open cell in row 6), and then row 7 would be left with no open cell for its marker. So J4 can't be the marker there.
32. After that, place a marker at B4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column B, and its region.
33. Now, every open cell in row 6 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.
34. From there, place a marker at A7. Row 7 has exactly one open cell left. Placing here clears the rest of row 7, column A, and its region.
35. Following that, place a marker at C10. Region has exactly one open cell left. Placing here clears the rest of row 10, column C, and its region.
36. First, place a marker at J6. Column 10 has exactly one open cell left. Placing here clears the rest of row 6, column J, and its region.
37. Next, every remaining candidate in this region touches G8 and G9, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
38. Then, place a marker at E9. Region has exactly one open cell left. Placing here clears the rest of row 9, column E, and its region.
39. After that, place a marker at G5. Region has exactly one open cell left.
40. Finally, place a marker at H8. Region has exactly one open cell left.

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