An Expert 10×10 Puzzle Solved with Forced Chain in 27 Steps

This 10×10 board rates as expert (difficulty score 1208). 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 row 1, so its marker has to land there; that clears every other open cell in the row.
2. Next, place a marker at H2. Region has exactly one open cell left. Placing here clears the rest of row 2, column H, and its region. Placing here also clears its 3 touching neighbours.
3. Then, place a marker at J1. Region has exactly one open cell left. Placing here clears the rest of row 1, column J, and its region.
4. After 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.
5. Now, 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.
6. From there, every open cell left in this region sits in row 8, so its marker has to land there; that clears every other open cell in the row.
7. Following that, every open cell left in this region sits in row 9, so its marker has to land there; that clears every other open cell in the row.
8. First, every open cell left in this region sits in row 10, so its marker has to land there; that clears every other open cell in the row.
9. Next, every remaining candidate in this region touches B6 and C6 and B8 and C8, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
10. Then, every remaining candidate in this region touches B10, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
11. After that, if A3 were the marker, it would force D8 (the last open cell in region 8), then B7 (the last open cell in region 7), and row 9 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 B3 it would force D4 (the last open cell in region 1), then C7 (the last open cell in region 7), then A8 (the last open cell in region 8), and row 9 would be left with no open cell for its marker.
13. From there, marking C3 would force A4 (the last open cell in region 1), then B7 (the last open cell in region 7), and then row 9 would be left with no open cell for its marker. So C3 can't be the marker there.
14. Following that, if D3 were the marker, it would force A8 (the last open cell in region 8), then C7 (the last open cell in region 7), and row 9 would end up with no open cell for its marker, which can't happen. So it can't go there.
15. First, marking F3 would force I5 (the last open cell in row 5), then E6 (the last open cell in column E), and then column G would be left with no open cell for its marker. So F3 can't be the marker there.
16. Next, place a marker at E3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column E, and its region. Placing here also clears its 1 touching neighbour.
17. Then, every open cell in column F 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.
18. After that, every open cell in column G 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.
19. Now, every remaining candidate in this region touches F5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
20. From there, place a marker at F6. Region has exactly one open cell left. Placing here clears the rest of row 6, column F, and its region. Placing here also clears its 1 touching neighbour.
21. Following that, place a marker at G4. Region has exactly one open cell left. Placing here clears the rest of row 4, column G, and its region.
22. First, place a marker at I5. Region has exactly one open cell left.
23. Next, that one can't be the marker: at C7 it would force A8 (the last open cell in region 8), and row 9 would be left with no open cell for its marker.
24. Then, 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 1 touching neighbour.
25. After that, 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. Placing here also clears its 1 touching neighbour.
26. Now, place a marker at A9. Region has exactly one open cell left. Placing here clears the rest of row 9, column A, and its region.
27. Finally, place a marker at C10. Region has exactly one open cell left.

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