An Expert 8×8 Puzzle Solved with Forced Chain in 23 Steps

This 8×8 board rates as expert (difficulty score 1305). Solving it from scratch takes 23 logical steps, using 8 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line 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 A, so its marker has to land there; that clears every other open cell in the column.
2. Next, every remaining candidate in this region touches F7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, marking D1 would force C5, C6 and C7 cleared (every open cell in column B belongs to the same region, so that region's marker has to be somewhere in this column), then C8 (the last open cell in column C), then F2, G2, F3, and 3 more cleared (every open cell in column E belongs to the same region, so that region's marker has to be somewhere in this column), and the chain continues, and then row 3 would be left with no open cell for its marker. So D1 can't be the marker there.
4. After that, if E1 were the marker, it would force C5, C6, D6, and 1 more cleared (every open cell in column B belongs to the same region, so that region's marker has to be somewhere in this column), then C8 (the last open cell in column C), then G2, F3, G3, and 2 more cleared (every open cell in column D belongs to the same region, so that region's marker has to be somewhere in this column), and the chain continues, and row 3 would end up with no open cell for its marker, which can't happen. So it can't go there.
5. Now, that one can't be the marker: at F1 it would force B3, C3, D3, and 2 more 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 G3 (the last open cell in row 3), then E8 (the last open cell in region 8), and region 7 would be left with no open cell for its marker.
6. From there, together, two regions have open cells only in columns G and H; 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.
7. Following that, every remaining candidate in this region touches E7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
8. First, if H1 were the marker, it would force G5 (the last open cell in region 6), then A8, B8, C8, and 1 more cleared (every open cell left in this region sits in row 8, so its marker has to land there), then B6, C6, D6, and 2 more cleared (together, two regions have open cells only in rows 6 and 7), and the chain continues, and column D would end up with no open cell for its marker, which can't happen. So it can't go there.
9. Next, marking B2 would force G1 (the last open cell in row 1), then F3 (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.
10. Then, if C2 were the marker, it would force G1 (the last open cell in row 1), then F3 (the last open cell in row 3), then E8 (the last open cell in region 8), and region 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
11. After that, that one can't be the marker: at D2 it would force G1 (the last open cell in row 1), then F3 (the last open cell in row 3), then E8 (the last open cell in region 8), and region 7 would be left with no open cell for its marker.
12. Now, marking G1 would force E2 (the last open cell in row 2), then C4 cleared (every open cell in row 3 belongs to the same region, so that region's marker has to be somewhere in this row), then B4 (the last open cell in row 4), and then row 3 would be left with no open cell for its marker. So G1 can't be the marker there.
13. From there, every open cell in row 1 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.
14. Following that, if F2 were the marker, it would force E8 (the last open cell in region 8), and region 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
15. First, marking H2 would force G5 (the last open cell in region 6), then F3 (the last open cell in row 3), then E8 (the last open cell in region 8), and then region 7 would be left with no open cell for its marker. So H2 can't be the marker there.
16. Next, 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.
17. Then, place a marker at H3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column H, and its region.
18. After that, place a marker at G5. Region has exactly one open cell left. Placing here clears the rest of row 5, column G, and its region. Placing here also clears its 1 touching neighbour.
19. Now, place a marker at F8. Region has exactly one open cell left. Placing here clears the rest of row 8, column F, and its region.
20. From there, place a marker at D7. Region 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.
21. Following that, place a marker at A6. Region has exactly one open cell left. Placing here clears the rest of row 6, column A, and its region.
22. First, place a marker at B4. Region has exactly one open cell left. Placing here clears the rest of row 4, column B, and its region.
23. Finally, place a marker at C1. Region has exactly one open cell left.

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