An Expert 8×8 Puzzle Solved with Forced Chain in 25 Steps

This 8×8 board rates as expert (difficulty score 1302). Solving it from scratch takes 25 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 5, so its marker has to land there; that clears every other open cell in the row.
2. Next, every open cell in column H 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.
3. Then, every remaining candidate in this region touches D4 and D6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
4. After that, every remaining candidate in this region touches C7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
5. Now, that one can't be the marker: at C1 it would force B3 and B4 cleared (every open cell left in this region sits in column B, so its marker has to land there), then B7 cleared (every remaining candidate in this region touches B7), then E4 cleared (every remaining candidate in this region touches E4), and the chain continues, and row 3 would be left with no open cell for its marker.
6. From there, marking D1 would force B2, B3, B4, and 1 more cleared (every open cell in column A belongs to the same region, so that region's marker has to be somewhere in this column), then C6 and C8 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 B7 cleared (every remaining candidate in this region touches B7), 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.
7. Following that, if E1 were the marker, it would force B2, C2, B3, and 2 more cleared (every open cell in column A belongs to the same region, so that region's marker has to be somewhere in this column), then C6 and C8 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 B7 cleared (every remaining candidate in this region touches B7), 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.
8. First, that one can't be the marker: at F1 it would force G3 (the last open cell in column G), and region 4 would be left with no open cell for its marker.
9. Next, if H1 were the marker, it would force G3 (the last open cell in column G), then E2 (the last open cell in region 4), then A4 (the last open cell in region 1), and region 3 would end up with no open cell for its marker, which can't happen. So it can't go there.
10. Then, 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.
11. After that, every remaining candidate in this region touches B2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
12. Now, if C2 were the marker, it would force A1 (the last open cell in region 1), then E3 (the last open cell in region 4), and column G 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: place it at D2, and region 4 would be left with no open cell for its marker.
14. Following that, if F2 were the marker, column G would end up with no open cell for its marker, which can't happen. So it can't go there.
15. First, that one can't be the marker: at G2 it would force B3 (the last open cell in row 3), then A1 (the last open cell in region 1), then H4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), and the chain continues, and column C would be left with no open cell for its marker.
16. Next, place a marker at G3. Column 7 has exactly one open cell left. Placing here clears the rest of row 3, column G, and its region. Placing here also clears its 2 touching neighbours.
17. Then, place a marker at E2. Region has exactly one open cell left. Placing here clears the rest of row 2, column E, and its region.
18. After that, every open cell in row 4 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.
19. Now, place a marker at A1. Column 1 has exactly one open cell left. Placing here clears the rest of row 1, column A, and its region.
20. From there, 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.
21. Following that, place a marker at D5. Column 4 has exactly one open cell left. Placing here clears the rest of row 5, column D, and its region. Placing here also clears its 2 touching neighbours.
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. Next, place a marker at C8. Region has exactly one open cell left. Placing here clears the rest of row 8, column C, and its region.
24. Then, place a marker at F7. Region has exactly one open cell left. Placing here clears the rest of row 7, column F, and its region.
25. 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. ∎