An Expert 8×8 Puzzle Solved with Forced Chain in 22 Steps

This 8×8 board rates as expert (difficulty score 1139). Solving it from scratch takes 22 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, 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, together, two regions have open cells only in rows 1 and 2; with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
3. Then, together, two regions have open cells only in columns A and B; 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.
4. After that, every open cell left in this region sits in row 2, so its marker has to land there; that clears every other open cell in the row.
5. Now, every remaining candidate in this region touches C3 and D3, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
6. From there, every remaining candidate in this region touches A6 and B6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
7. Following that, if H1 were the marker, it would force E4, C5, E5, and 4 more cleared (every open cell in row 6 belongs to the same region, so that region's marker has to be somewhere in this row), then G8 (the last open cell in row 8), then F4 (the last open cell in region 4), 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.
8. First, place a marker at G1. Region has exactly one open cell left. Placing here clears the rest of row 1, column G, and its region.
9. Next, together, two regions have open cells only in columns H and F; 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.
10. Then, that one can't be the marker: at D2 it would force C4 (the last open cell in region 2), then H3 (the last open cell in region 4), then A5 (the last open cell in region 5), and column F would be left with no open cell for its marker.
11. After that, place a marker at C2. Region 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.
12. Now, every remaining candidate in this region touches E4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
13. From there, that one can't be the marker: place it at E3, and region 4 would be left with no open cell for its marker.
14. Following that, every open cell left in this region sits in column D, so its marker has to land there; that clears every other open cell in the column.
15. First, every remaining candidate in this region touches E5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
16. Next, that one can't be the marker: at H3 it would force F5 (the last open cell in region 7), and row 6 would be left with no open cell for its marker.
17. Then, 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.
18. After that, place a marker at D5. Region has exactly one open cell left. Placing here clears the rest of row 5, column D, and its region. Placing here also clears its 1 touching neighbour.
19. Now, place a marker at A3. Region has exactly one open cell left. Placing here clears the rest of row 3, column A, and its region.
20. From there, place a marker at B7. Region has exactly one open cell left.
21. Following that, place a marker at E8. Region has exactly one open cell left. Placing here clears the rest of row 8, column E, and its region.
22. 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. ∎