An Expert 10×10 Puzzle Solved with Forced Chain in 25 Steps

This 10×10 board rates as expert (difficulty score 1182). Solving it from scratch takes 25 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Paired regions, 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 C, so its marker has to land there; that clears every other open cell in the column.
2. Next, together, two regions have open cells only in columns A and B; 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.
3. Then, place a marker at D4. Region has exactly one open cell left. Placing here clears the rest of row 4, column D, and its region. Placing here also clears its 2 touching neighbours.
4. After that, place a marker at H5. Region has exactly one open cell left. Placing here clears the rest of row 5, column H, and its region. Placing here also clears its 2 touching neighbours.
5. Now, 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.
6. From there, together, two regions have open cells only in rows 1 and 2; with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
7. Following that, together, two regions have open cells only in columns I and J; 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.
8. First, every remaining candidate in this region touches A6 and A7, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
9. Next, that one can't be the marker: at F1 it would force I3 and J3 cleared (every remaining candidate in this region touches I3 and J3), then J6 (the last open cell in region 5), then I2 (the last open cell in region 3), and the chain continues, and row 3 would be left with no open cell for its marker.
10. Then, marking G1 would force I3 and J3 cleared (every remaining candidate in this region touches I3 and J3), then J6 (the last open cell in region 5), then I2 (the last open cell in region 3), and the chain continues, and then region 9 would be left with no open cell for its marker. So G1 can't be the marker there.
11. 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.
12. Now, marking J1 would force E2 (the last open cell in region 2), then I3 (the last open cell in region 5), then F8 (the last open cell in region 4), and the chain continues, and then column A would be left with no open cell for its marker. So J1 can't be the marker there.
13. From there, that one can't be the marker: place it at J2, and column I would be left with no open cell for its marker.
14. Following that, 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.
15. First, if G3 were the marker, it would force J6 (the last open cell in region 5), then B7 (the last open cell in region 6), and row 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
16. Next, every open cell left in this region sits in column F, so its marker has to land there; that clears every other open cell in the column.
17. Then, that one can't be the marker: at I1 it would force E2 (the last open cell in region 2), then F8 (the last open cell in region 4), then G10 (the last open cell in region 9), and the chain continues, and column A would be left with no open cell for its marker.
18. After that, 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 1 touching neighbour.
19. Now, place a marker at E1. Region has exactly one open cell left.
20. From there, place a marker at J6. Region has exactly one open cell left. Placing here clears the rest of row 6, column J, and its region.
21. Following that, place a marker at B7. Region has exactly one open cell left. Placing here clears the rest of row 7, column B, and its region. Placing here also clears its 2 touching neighbours.
22. First, place a marker at C9. Region has exactly one open cell left. Placing here clears the rest of row 9, column C, and its region.
23. Next, place a marker at A10. Region has exactly one open cell left. Placing here clears the rest of row 10, column A, and its region.
24. Then, 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.
25. Finally, place a marker at F3. Region has exactly one open cell left.

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