An Expert 9×9 Puzzle Solved with Forced Chain in 20 Steps

This 9×9 board rates as expert (difficulty score 1160). 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 column H, so its marker has to land there; that clears every other open cell in the column.
2. Next, every remaining candidate in this region touches G1 and I1 and G2 and I2, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
3. Then, marking A1 would force H2 (the last open cell in region 4), and then region 3 would be left with no open cell for its marker. So A1 can't be the marker there.
4. After that, if B1 were the marker, it would force H2 (the last open cell in region 4), and region 3 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 B3, 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 C1 it would force H2 (the last open cell in region 4), and region 3 would be left with no open cell for its marker.
7. Following that, every remaining candidate in this region touches D3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
8. First, marking D1 would force H2 (the last open cell in region 4), then C3 (the last open cell in region 2), then A4 (the last open cell in region 1), and the chain continues, and then region 8 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 H2 (the last open cell in region 4), then C3 (the last open cell in region 2), then A4 (the last open cell in region 1), and region 6 would end up with no open cell for its marker, which can't happen. So it can't go there.
11. After that, if H1 were the marker, it would force A2, B2, A3, and 3 more cleared (together, two regions have open cells only in rows 2 and 3), then D4, E4, F4, and 2 more cleared (every open cell left in this region sits in row 4, so its marker has to land there), then E5 (the last open cell in region 6), and the chain continues, and region 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
12. Now, place a marker at H2. Region has exactly one open cell left. Placing here clears the rest of row 2, column H, and its region. Placing here also clears its 2 touching neighbours.
13. From there, place a marker at F1. Region has exactly one open cell left. Placing here clears the rest of row 1, column F, and its region.
14. Following that, place a marker at C3. Region has exactly one open cell left. Placing here clears the rest of row 3, column C, and its region. Placing here also clears its 2 touching neighbours.
15. First, place a marker at A4. Region has exactly one open cell left. Placing here clears the rest of row 4, column A, and its region. Placing here also clears its 1 touching neighbour.
16. Next, place a marker at E5. Region has exactly one open cell left. Placing here clears the rest of row 5, column E, and its region. Placing here also clears its 1 touching neighbour.
17. Then, place a marker at G6. Region has exactly one open cell left. Placing here clears the rest of row 6, column G, and its region.
18. After that, place a marker at B9. Region has exactly one open cell left. Placing here clears the rest of row 9, column B, 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. Finally, place a marker at I8. Region has exactly one open cell left.

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