An Expert 8×8 Puzzle Solved with Forced Chain in 26 Steps

This 8×8 board rates as expert (difficulty score 1401). Solving it from scratch takes 26 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 7, so its marker has to land there; that clears every other open cell in the row.
2. Next, 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.
3. Then, every remaining candidate in this region touches G5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
4. After that, every remaining candidate in this region touches B7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
5. Now, every remaining candidate in this region touches D6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
6. From there, marking A1 would force B6 and B8 cleared (every open cell left in this region sits in column B, so its marker has to land there), then C8 (the last open cell in region 7), then E7 (the last open cell in region 8), and then region 6 would be left with no open cell for its marker. So A1 can't be the marker there.
7. Following that, if B1 were the marker, it would force A8 cleared (every open cell left in this region sits in column A, so its marker has to land there), then C8 (the last open cell in region 7), then E7 (the last open cell in region 8), and region 6 would end up with no open cell for its marker, which can't happen. So it can't go there.
8. First, that one can't be the marker: at C1 it would force F2, H3, D4, and 3 more cleared (every open cell in column G belongs to the same region, so that region's marker has to be somewhere in this column), then B6 cleared (together, two regions have open cells only in columns B and A), then E2, E3 and E7 cleared (every open cell left in this region sits in column E, so its marker has to land there), and the chain continues, and row 2 would be left with no open cell for its marker.
9. Next, if E1 were the marker, it would force A6, F6, G6, and 1 more cleared (every open cell left in this region sits in row 6, so its marker has to land there), then H3, D4, F4, and 1 more cleared (every open cell in column G belongs to the same region, so that region's marker has to be somewhere in this column), then D7 (the last open cell in column D), and the chain continues, and region 4 would end up with no open cell for its marker, which can't happen. So it can't go there.
10. Then, marking G1 would force A3 and A4 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 B3 cleared (every remaining candidate in this region touches B3), then H3 (the last open cell in row 3), and the chain continues, and then row 6 would be left with no open cell for its marker. So G1 can't be the marker there.
11. 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.
12. Now, if H1 were the marker, it would force D7 (the last open cell in column D), then C2 (the last open cell in column C), then G3 (the last open cell in row 3), and the chain continues, and row 6 would end up with no open cell for its marker, which can't happen. So it can't go there.
13. From there, that one can't be the marker: at A2 it would force F1 (the last open cell in row 1), then G3 (the last open cell in row 3), then B4 (the last open cell in row 4), and the chain continues, and column D would be left with no open cell for its marker.
14. Following that, marking B2 would force F1 (the last open cell in row 1), then G3 (the last open cell in row 3), and then row 4 would be left with no open cell for its marker. So B2 can't be the marker there.
15. First, that one can't be the marker: place it at D2, and row 1 would be left with no open cell for its marker.
16. Next, marking E2 would immediately leave row 1 with no open cell for its marker, so it can't be the marker there.
17. Then, that one can't be the marker: at G2 it would force D1 (the last open cell in row 1), then H4, H5 and H6 cleared (every open cell in column F belongs to the same region, so that region's marker has to be somewhere in this column), and column H would be left with no open cell for its marker.
18. After that, marking D1 would force H2 (the last open cell in row 2), then B3 (the last open cell in row 3), then G4 (the last open cell in row 4), and the chain continues, and then row 5 would be left with no open cell for its marker. So D1 can't be the marker there.
19. Now, place a marker at F1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column F, and its region.
20. From there, place a marker at C2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column C, and its region. Placing here also clears its 1 touching neighbour.
21. Following that, place a marker at G3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column G, and its region. Placing here also clears its 1 touching neighbour.
22. First, 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.
23. Next, place a marker at A8. Region has exactly one open cell left.
24. Then, place a marker at D7. Column 4 has exactly one open cell left. Placing here clears the rest of row 7, column D, and its region. Placing here also clears its 1 touching neighbour.
25. After that, place a marker at E5. Region has exactly one open cell left. Placing here clears the rest of row 5, column E, and its region.
26. Finally, place a marker at H6. Region has exactly one open cell left.

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