An Expert 8×8 Puzzle Solved with Forced Chain in 18 Steps

This 8×8 board rates as expert (difficulty score 1145). Solving it from scratch takes 18 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Line confinement, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, 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. Placing here also clears its 1 touching neighbour.
2. Next, every remaining candidate in this region touches B6 and C6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
3. Then, if B1 were the marker, it would force C7 (the last open cell in region 4), then H6 (the last open cell in region 7), and region 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
4. After that, that one can't be the marker: at C1 it would force B5 (the last open cell in region 4), then G4 and H4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), then G3 and H3 cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), and row 3 would be left with no open cell for its marker.
5. Now, that one can't be the marker: at F1 it would force H6, H7 and G7 cleared (together, two regions have open cells only in columns H and G), then D7 (the last open cell in region 7), then B5 (the last open cell in region 4), and region 6 would be left with no open cell for its marker.
6. From there, if H1 were the marker, it would force C7, E7 and F7 cleared (every open cell left in this region sits in row 7, so its marker has to land there), then B5 (the last open cell in region 4), then D6 and E6 cleared (every open cell left in this region sits in row 6, so its marker has to land there), and region 6 would end up with no open cell for its marker, which can't happen. So it can't go there.
7. Following that, marking B2 would force C7 (the last open cell in region 4), then H6 (the last open cell in region 7), and then region 5 would be left with no open cell for its marker. So B2 can't be the marker there.
8. First, if E1 were the marker, it would force G3, H3 and G4 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 B4, C4, D4, and 2 more 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 H4 (the last open cell in row 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.
9. Next, marking G1 would force D3, E3 and F3 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 B4, C4, D4, and 3 more 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 H4 (the last open cell in row 4), and the chain continues, and then row 6 would be left with no open cell for its marker. So G1 can't be the marker there.
10. Then, 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.
11. After that, every open cell in row 2 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.
12. Now, every open cell in row 3 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.
13. From there, place a marker at H4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column H, and its region.
14. Following that, place a marker at G7. Region has exactly one open cell left. Placing here clears the rest of row 7, column G, and its region.
15. First, place a marker at F2. Region has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region.
16. Next, 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.
17. Then, place a marker at C3. Region has exactly one open cell left.
18. Finally, place a marker at E6. Region has exactly one open cell left.

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