An Expert 7×7 Puzzle Solved with Forced Chain in 20 Steps

This 7×7 board rates as expert (difficulty score 1244). Solving it from scratch takes 20 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region 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 4, so its marker has to land there; that clears every other open cell in the row.
2. Next, 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.
3. Then, every remaining candidate in this region touches E3 and E5, 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 E6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
5. Now, marking A1 would force D2 (the last open cell in region 2), then C6 (the last open cell in column C), and then column B would be left with no open cell for its marker. So A1 can't be the marker there.
6. From there, that one can't be the marker: place it at C1, and region 2 would be left with no open cell for its marker.
7. Following that, marking D1 would force F2, F3 and F5 cleared (every open cell in column G belongs to the same region, so that region's marker has to be somewhere in this column), then F6 cleared (together, two regions have open cells only in columns E and F), then B5 and B6 cleared (every remaining candidate in this region touches B5 and B6), and the chain continues, and then column A would be left with no open cell for its marker. So D1 can't be the marker there.
8. First, if E1 were the marker, it would force D3, F3, D5, 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), then D6 and F6 cleared (together, two regions have open cells only in columns D and F), then B5 and B6 cleared (every remaining candidate in this region touches B5 and B6), and the chain continues, and column A would end up with no open cell for its marker, which can't happen. So it can't go there.
9. Next, that one can't be the marker: at F1 it would force D3 and D5 cleared (every open cell in column G belongs to the same region, so that region's marker has to be somewhere in this column), then D6 cleared (together, two regions have open cells only in columns D and E), then B5 and B6 cleared (every remaining candidate in this region touches B5 and B6), and the chain continues, and column A would be left with no open cell for its marker.
10. Then, that one can't be the marker: place it at A2, and column B would be left with no open cell for its marker.
11. After that, marking B2 would immediately leave region 3 with no open cell for its marker, so it can't be the marker there.
12. Now, if C2 were the marker, it would force G1 (the last open cell in region 2), then A3 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 F3 (the last open cell in row 3), and the chain continues, and column A would end up with no open cell for its marker, which can't happen. So it can't go there.
13. From there, marking G1 would force E2 (the last open cell in row 2), then D6 and F6 cleared (together, two regions have open cells only in columns D and F), then B5 and B6 cleared (every remaining candidate in this region touches B5 and B6), and the chain continues, and then column B would be left with no open cell for its marker. So G1 can't be the marker there.
14. Following that, place a marker at D2. Region has exactly one open cell left. Placing here clears the rest of row 2, column D, and its region. Placing here also clears its 1 touching neighbour.
15. First, place a marker at B1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column B, and its region.
16. Next, 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.
17. Then, place a marker at C6. Column 3 has exactly one open cell left. Placing here clears the rest of row 6, column C, and its region.
18. After that, place a marker at G5. Column 7 has exactly one open cell left. Placing here clears the rest of row 5, column G, and its region. Placing here also clears its 1 touching neighbour.
19. Now, place a marker at E4. Region has exactly one open cell left. Placing here clears the rest of row 4, column E, and its region.
20. Finally, place a marker at F7. Region has exactly one open cell left.

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