An Expert 10×10 Puzzle Solved with Forced Chain in 34 Steps

This 10×10 board rates as expert (difficulty score 1643). Solving it from scratch takes 34 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, if B1 were the marker, it would force D2, E2, F2, and 2 more cleared (every open cell left in this region sits in row 2, so its marker has to land there), then E3, F3, G3, and 3 more cleared (every open cell left in this region sits in row 3, so its marker has to land there), then C5, C9, C10, and 3 more cleared (together, two regions have open cells only in columns C and D), and the chain continues, and region 10 would end up with no open cell for its marker, which can't happen. So it can't go there.
2. Next, that one can't be the marker: at C1 it would force A2, F2, G2, and 1 more cleared (every open cell left in this region sits in row 2, so its marker has to land there), then D4, D5, D9, and 1 more cleared (every open cell left in this region sits in column D, so its marker has to land there), then A8, A9, A10, and 3 more 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.
3. Then, marking D1 would force A2, B2, F2, and 2 more cleared (every open cell left in this region sits in row 2, so its marker has to land there), then C4, C5, C9, and 1 more cleared (every open cell left in this region sits in column C, so its marker has to land there), then A8, A9, A10, and 3 more 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 D1 can't be the marker there.
4. After that, that one can't be the marker: at F1 it would force A2, B2, C2, and 2 more cleared (every open cell left in this region sits in row 2, so its marker has to land there), then A3, 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 G4, H4, I4, and 1 more cleared (every open cell left in this region sits in row 4, so its marker has to land there), and the chain continues, and column G would be left with no open cell for its marker.
5. Now, marking G1 would force A2, B2, C2, and 2 more cleared (every open cell left in this region sits in row 2, so its marker has to land there), then A3, H3, I3, and 1 more cleared (every open cell left in this region sits in row 3, so its marker has to land there), then H4, I4 and J4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), and the chain continues, and then column J would be left with no open cell for its marker. So G1 can't be the marker there.
6. From there, if H1 were the marker, it would force J2 (the last open cell in region 4), then A3 and G3 cleared (every open cell left in this region sits in row 3, so its marker has to land there), then G4 and I4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), 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.
7. Following that, that one can't be the marker: at A2 it would force E1 cleared (every open cell left in this region sits in row 1, so its marker has to land there), then E3, F3, G3, and 3 more cleared (every open cell left in this region sits in row 3, so its marker has to land there), then E7, G7, H7, and 12 more cleared (every open cell in row 10 belongs to the same region, so that region's marker has to be somewhere in this row), and the chain continues, and region 7 would be left with no open cell for its marker.
8. First, marking B2 would force D3 (the last open cell in region 2), then E1 cleared (every open cell left in this region sits in row 1, so its marker has to land there), then C5, C9 and C10 cleared (every open cell left in this region sits in column C, so its marker has to land there), and the chain continues, and then region 10 would be left with no open cell for its marker. So B2 can't be the marker there.
9. Next, if C2 were the marker, it would force A1 and E1 cleared (every open cell left in this region sits in row 1, so its marker has to land there), then D4, D5, D9, and 1 more cleared (every open cell left in this region sits in column D, so its marker has to land there), then A8, A9, A10, and 3 more 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.
10. Then, that one can't be the marker: at D2 it would force A1 cleared (every open cell left in this region sits in row 1, so its marker has to land there), then C4, C5, C9, and 1 more cleared (every open cell left in this region sits in column C, so its marker has to land there), then A8, A9, A10, and 3 more 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.
11. After that, if F2 were the marker, it would force A1 cleared (every open cell left in this region sits in row 1, so its marker has to land there), then A3, H3, I3, and 1 more cleared (every open cell left in this region sits in row 3, so its marker has to land there), then G4, H4, I4, and 1 more cleared (every open cell left in this region sits in row 4, so its marker has to land there), 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.
12. Now, that one can't be the marker: at G2 it would force A1 cleared (every open cell left in this region sits in row 1, so its marker has to land there), then A3, I3 and J3 cleared (every open cell left in this region sits in row 3, so its marker has to land there), then H4, I4 and J4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), and the chain continues, and column J would be left with no open cell for its marker.
13. From there, marking H2 would force J1 (the last open cell in region 4), then A3, E3 and F3 cleared (every open cell left in this region sits in row 3, so its marker has to land there), then E4, F4 and I4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), and the chain continues, and then column G would be left with no open cell for its marker. So H2 can't be the marker there.
14. Following that, marking J1 would force E2 (the last open cell in row 2), then F4 (the last open cell in region 3), then A3 (the last open cell in region 1), and the chain continues, and then row 9 would be left with no open cell for its marker. So J1 can't be the marker there.
15. First, marking C3 would force A5, A6, A7, and 3 more cleared (every open cell left in this region sits in column A, so its marker has to land there), then D5, D9 and D10 cleared (every open cell left in this region sits in column D, so its marker has to land there), then B8, B9 and B10 cleared (every open cell left in this region sits in column B, so its marker has to land there), and then region 10 would be left with no open cell for its marker. So C3 can't be the marker there.
16. Next, if D3 were the marker, it would force C5, C9 and C10 cleared (every open cell left in this region sits in column C, so its marker has to land there), then I1 cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), then A8, A9, A10, and 3 more 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.
17. Then, marking A1 would force E2 cleared (every open cell left in this region sits in row 2, so its marker has to land there), then B3 (the last open cell in region 2), then E5, F5, G5, and 3 more cleared (every open cell left in this region sits in row 5, so its marker has to land there), and the chain continues, and then row 9 would be left with no open cell for its marker. So A1 can't be the marker there.
18. After that, if E1 were the marker, it would force B3 (the last open cell in region 2), then D4 (the last open cell in region 1), then A8, A9 and A10 cleared (every open cell left in this region sits in column A, so its marker has to land there), and the chain continues, and region 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
19. Now, place a marker at I1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column I, and its region.
20. From there, place a marker at E2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column E, and its region. Placing here also clears its 1 touching neighbour.
21. Following that, place a marker at F4. Region has exactly one open cell left. Placing here clears the rest of row 4, column F, and its region. Placing here also clears its 2 touching neighbours.
22. First, place a marker at A3. Region has exactly one open cell left. Placing here clears the rest of row 3, column A, and its region.
23. Next, every open cell in row 10 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.
24. Then, every open cell in row 9 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.
25. After that, every open cell in row 8 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.
26. Now, every remaining candidate in this region touches H6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
27. From there, every open cell left in this region sits in column J, so its marker has to land there; that clears every other open cell in the column.
28. Following that, every remaining candidate in this region touches C9 and D9, 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 B9. Region has exactly one open cell left. Placing here clears the rest of row 9, column B, and its region. Placing here also clears its 2 touching neighbours.
30. Next, place a marker at D8. Region has exactly one open cell left. Placing here clears the rest of row 8, column D, and its region.
31. Then, 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.
32. After that, place a marker at G6. Region has exactly one open cell left. Placing here clears the rest of row 6, column G, and its region.
33. Now, place a marker at J7. Region has exactly one open cell left.
34. Finally, place a marker at H10. Region has exactly one open cell left.

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