An Expert 8×8 Puzzle Solved with Forced Chain in 21 Steps

This 8×8 board rates as expert (difficulty score 1269). Solving it from scratch takes 21 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, marking A1 would force B3 (the last open cell in region 2), then C6 (the last open cell in region 6), and then region 8 would be left with no open cell for its marker. So A1 can't be the marker there.
2. Next, if B1 were the marker, it would force D6, E6, F6, and 2 more cleared (every open cell left in this region sits in row 6, so its marker has to land there), then A7, A8, C3, and 2 more cleared (together, two regions have open cells only in columns A and C), and region 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
3. Then, that one can't be the marker: at C1 it would force A7, A8, B7, and 1 more cleared (together, two regions have open cells only in columns A and B), and region 8 would be left with no open cell for its marker.
4. After that, if E1 were the marker, it would force C4, C5, C6, and 1 more cleared (together, two regions have open cells only in columns C and D), then A7, A8, B7, and 1 more cleared (together, two regions have open cells only in columns A and B), and region 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
5. Now, every remaining candidate in this region touches C2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
6. From there, that one can't be the marker: at F1 it would force B4, B5, B6, and 2 more cleared (every open cell left in this region sits in column B, so its marker has to land there), then D6, E6, G6, and 1 more cleared (every open cell left in this region sits in row 6, so its marker has to land there), then H5, E7, H7, and 2 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), and column H would be left with no open cell for its marker.
7. Following that, marking G1 would force B4, B5, B6, and 2 more cleared (every open cell left in this region sits in column B, so its marker has to land there), then D6, E6, F6, and 1 more cleared (every open cell left in this region sits in row 6, so its marker has to land there), then H5, E7, H7, and 2 more 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 then column H would be left with no open cell for its marker. So G1 can't be the marker there.
8. First, if H1 were the marker, it would force B4, B5, B6, and 2 more cleared (every open cell left in this region sits in column B, so its marker has to land there), then D6, E6, F6, and 1 more cleared (every open cell left in this region sits in row 6, so its marker has to land there), then E7, F7, E8, 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), and the chain continues, and column E would end up with no open cell for its marker, which can't happen. So it can't go there.
9. Next, place a marker at D1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column D, and its region. Placing here also clears its 1 touching neighbour.
10. Then, that one can't be the marker: at A2 it would force C6, C8, E7, and 1 more cleared (together, two regions have open cells only in columns C and E), then B7 and B8 cleared (every open cell left in this region sits in column B, so its marker has to land there), and region 8 would be left with no open cell for its marker.
11. After that, that one can't be the marker: at G2 it would force H5, H6, E7, and 3 more 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.
12. Now, marking H2 would force G4, G5, G6, and 4 more 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 then column G would be left with no open cell for its marker. So H2 can't be the marker there.
13. From there, place a marker at F2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region. Placing here also clears its 1 touching neighbour.
14. Following that, 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.
15. First, 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.
16. Next, place a marker at E6. Column 5 has exactly one open cell left. Placing here clears the rest of row 6, column E, and its region.
17. Then, place a marker at B5. Region has exactly one open cell left. Placing here clears the rest of row 5, column B, and its region. Placing here also clears its 2 touching neighbours.
18. After that, 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.
19. Now, place a marker at H4. Region has exactly one open cell left. Placing here clears the rest of row 4, column H, and its region.
20. From there, 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.
21. Finally, place a marker at G7. Region has exactly one open cell left.

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