How to solve this 9×9 logic puzzle

9×9 · severe · 24 steps

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This 9×9 board rates as severe (difficulty score 1208). Solving it from scratch takes 34 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Paired regions, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

First, every open cell left in this region sits in row 1, so its marker has to land there; that clears every other open cell in the row.
Next, every open cell left in this region sits in column E, so its marker has to land there; that clears every other open cell in the column.
Then, together, two regions have open cells only in rows 8 and 9; with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
After that, together, two regions have open cells only in rows 6 and 7; with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
Now, every remaining candidate in this region touches H2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
From there, every remaining candidate in this region touches B4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Following that, marking G1 would force H4 (the last open cell in column H), then I2 (the last open cell in region 3), then F7 (the last open cell in column F), and then column D would be left with no open cell for its marker. So G1 can't be the marker there.
First, every remaining candidate in this region touches I2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Next, that one can't be the marker: place it at A2, and region 6 would be left with no open cell for its marker.
Then, marking B2 would immediately leave region 6 with no open cell for its marker, so it can't be the marker there.
After that, if C2 were the marker, it would force D6 (the last open cell in row 6), then F3 (the last open cell in column F), and column G would end up with no open cell for its marker, which can't happen. So it can't go there.
Now, if F2 were the marker, it would force I1 (the last open cell in column I), then H4 (the last open cell in column H), and column G would end up with no open cell for its marker, which can't happen. So it can't go there.
From there, that one can't be the marker: at G2 it would force I1 (the last open cell in region 2), then H4 (the last open cell in column H), then F7 (the last open cell in column F), and column D would be left with no open cell for its marker.
Following that, place a marker at D2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column D, and its region.
First, place a marker at F7. Region has exactly one open cell left. Placing here clears the rest of row 7, column F, and its region. Placing here also clears its 1 touching neighbour.
Next, place a marker at C6. Region has exactly one open cell left. Placing here clears the rest of row 6, column C, and its region. Placing here also clears its 1 touching neighbour.
Then, place a marker at A5. Region has exactly one open cell left. Placing here clears the rest of row 5, column A, and its region.
After that, place a marker at E9. Region has exactly one open cell left. Placing here clears the rest of row 9, column E, and its region.
Now, place a marker at B8. Region has exactly one open cell left.
From there, 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.
Following that, every remaining candidate in this region touches H4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
First, place a marker at G4. Region has exactly one open cell left. Placing here clears the rest of row 4, column G, and its region. Placing here also clears its 1 touching neighbour.
Next, place a marker at I3. Region has exactly one open cell left. Placing here clears the rest of row 3, column I, and its region.
Finally, place a marker at H1. Region has exactly one open cell left.

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