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

This 8×8 board rates as expert (difficulty score 1228). 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, 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 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 C4, 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 and B8, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
5. Now, every remaining candidate in this region touches C6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
6. From there, if B1 were the marker, it would force C5 (the last open cell in region 6), then D3 (the last open cell in region 3), and region 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
7. Following that, 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 D3 (the last open cell in region 3), and region 7 would be left with no open cell for its marker.
8. First, marking D1 would force C3 and C5 cleared (every open cell left in this region sits in column C, so its marker has to land there), then B5 and B6 cleared (every open cell left in this region sits in column B, so its marker has to land there), and then region 6 would be left with no open cell for its marker. So D1 can't be the marker there.
9. Next, that one can't be the marker: at F1 it would force G3 (the last open cell in region 4), and row 2 would be left with no open cell for its marker.
10. Then, marking G1 would immediately leave row 2 with no open cell for its marker, so it can't be the marker there.
11. After that, marking B2 would force D3 (the last open cell in region 3), then F4 (the last open cell in region 4), and then column G would be left with no open cell for its marker. So B2 can't be the marker there.
12. Now, if H1 were the marker, it would force F2 cleared (every open cell left in this region sits in row 2, so its marker has to land there), then E3, D4 and E4 cleared (together, two regions have open cells only in rows 3 and 4), then D5 and D6 cleared (together, two regions have open cells only in rows 5 and 6), and the chain continues, and row 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
13. From there, place a marker at E1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column E, and its region. Placing here also clears its 1 touching neighbour.
14. Following that, marking H2 would force G5 (the last open cell in column G), then F3 (the last open cell in region 4), then B4 (the last open cell in region 3), and then region 6 would be left with no open cell for its marker. So H2 can't be the marker there.
15. First, place a marker at G2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column G, and its region. Placing here also clears its 1 touching neighbour.
16. Next, place a marker at F5. Column 6 has exactly one open cell left. Placing here clears the rest of row 5, column F, and its region.
17. Then, place a marker at B6. Region has exactly one open cell left. Placing here clears the rest of row 6, column B, and its region. Placing here also clears its 2 touching neighbours.
18. After that, place a marker at A8. Region has exactly one open cell left. Placing here clears the rest of row 8, column A, and its region.
19. Now, 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.
20. From there, place a marker at C3. Region has exactly one open cell left.
21. Finally, place a marker at H4. Region has exactly one open cell left.

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