An Expert 10×10 Puzzle Solved with Forced Chain in 26 Steps

This 10×10 board rates as expert (difficulty score 1279). Solving it from scratch takes 26 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, together, two regions have open cells only in columns J and I; 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.
2. Next, every open cell left in this region sits in column H, so its marker has to land there; that clears every other open cell in the column.
3. Then, every open cell left in this region sits in column G, so its marker has to land there; that clears every other open cell in the column.
4. After that, 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.
5. Now, together, two regions have open cells only in columns E and D; 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.
6. From there, every remaining candidate in this region touches E3 and G3 and E4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
7. Following that, marking A1 would force G2 (the last open cell in region 3), then D4 (the last open cell in region 2), and then column F would be left with no open cell for its marker. So A1 can't be the marker there.
8. First, if B1 were the marker, it would force G2 (the last open cell in region 3), then D4 (the last open cell in region 2), and column F would end up with no open cell for its marker, which can't happen. So it can't go there.
9. Next, that one can't be the marker: at C1 it would force G2 (the last open cell in region 3), then D4 (the last open cell in region 2), and column F would be left with no open cell for its marker.
10. Then, marking J1 would force G2 (the last open cell in region 3), then D4 (the last open cell in region 2), and then column F would be left with no open cell for its marker. So J1 can't be the marker there.
11. After that, place a marker at G1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column G, and its region.
12. Now, that one can't be the marker: at A2 it would force D4 (the last open cell in region 2), then F3 (the last open cell in region 5), and region 4 would be left with no open cell for its marker.
13. From there, marking B2 would force D4 (the last open cell in region 2), then F3 (the last open cell in region 5), and then region 4 would be left with no open cell for its marker. So B2 can't be the marker there.
14. Following that, if C2 were the marker, it would force D4 (the last open cell in region 2), then F3 (the last open cell in region 5), and region 4 would end up with no open cell for its marker, which can't happen. So it can't go there.
15. First, if I2 were the marker, it would force D4 (the last open cell in region 2), then F3 (the last open cell in region 5), and region 1 would end up with no open cell for its marker, which can't happen. So it can't go there.
16. Next, that one can't be the marker: at J2 it would force D4 (the last open cell in region 2), then F3 (the last open cell in region 5), and region 1 would be left with no open cell for its marker.
17. Then, 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.
18. After 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.
19. Now, place a marker at D9. Region has exactly one open cell left. Placing here clears the rest of row 9, column D, and its region. Placing here also clears its 2 touching neighbours.
20. From there, place a marker at B10. Region has exactly one open cell left. Placing here clears the rest of row 10, column B, and its region.
21. Following that, every open cell left in this region sits in row 3, so its marker has to land there; that clears every other open cell in the row.
22. First, 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.
23. Next, place a marker at I6. Region has exactly one open cell left. Placing here clears the rest of row 6, column I, and its region. Placing here also clears its 1 touching neighbour.
24. Then, place a marker at J3. Region has exactly one open cell left.
25. After that, place a marker at H8. Region has exactly one open cell left. Placing here clears the rest of row 8, column H, and its region.
26. Finally, place a marker at A7. Region has exactly one open cell left.

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