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

This 7×7 board rates as expert (difficulty score 1236). Solving it from scratch takes 20 logical steps, using 5 techniques: Single cell, Row/column exclusion, Line confinement, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, every open cell in row 1 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.
2. Next, every remaining candidate in this region touches B6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, every remaining candidate in this region touches A5, 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 E7, 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 B5 (the last open cell in region 5), then C2 (the last open cell in region 2), and then region 6 would be left with no open cell for its marker. So A1 can't be the marker there.
6. From there, if B1 were the marker, it would force A4 (the last open cell in region 2), and region 5 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 D7, F7 and G7 cleared (every open cell left in this region sits in row 7, so its marker has to land there), then A6 and G6 cleared (every open cell left in this region sits in row 6, so its marker has to land there), then B5 (the last open cell in region 5), and region 2 would be left with no open cell for its marker.
8. First, marking D1 would force A6, C6 and G6 cleared (every open cell left in this region sits in row 6, so its marker has to land there), then B5 (the last open cell in region 5), and then region 2 would be left with no open cell for its marker. So D1 can't be the marker there.
9. Next, every remaining candidate in this region touches F2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
10. Then, if E1 were the marker, it would force B3, A4 and B4 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 C4, D4, G4, and 1 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 F4 (the last open cell in row 4), and column G would end up with no open cell for its marker, which can't happen. So it can't go there.
11. After that, that one can't be the marker: at F1 it would force B3, A4 and B4 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 C4, D4, E4, 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), and row 4 would be left with no open cell for its marker.
12. Now, 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.
13. From there, marking B2 would force A6 (the last open cell in region 5), then D7 (the last open cell in region 7), then C5 (the last open cell in region 6), and the chain continues, and then row 3 would be left with no open cell for its marker. So B2 can't be the marker there.
14. Following that, if C2 were the marker, it would force D7 and F7 cleared (every open cell left in this region sits in row 7, so its marker has to land there), then A6 cleared (every open cell left in this region sits in row 6, so its marker has to land there), then B5 (the last open cell in region 5), and the chain continues, and row 3 would end up with no open cell for its marker, which can't happen. So it can't go there.
15. First, place a marker at E2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column E, and its region.
16. Next, place a marker at B3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column B, and its region.
17. Then, place a marker at A6. Region has exactly one open cell left. Placing here clears the rest of row 6, column A, and its region.
18. After that, 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.
19. Now, place a marker at C5. Region has exactly one open cell left. Placing here clears the rest of row 5, column C, and its region.
20. Finally, place a marker at F4. Region has exactly one open cell left.

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