A Hard 6×6 Puzzle Solved with Forced Chain in 15 Steps

This 6×6 board rates as hard (difficulty score 1081). Solving it from scratch takes 15 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line 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 remaining candidate in this region touches B3 and C3 and B5 and C5, 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 B6 (the last open cell in region 5), then C4 (the last open cell in region 6), 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 C4 (the last open cell in region 6), 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, marking D1 would force C6 (the last open cell in region 3), then B4 (the last open cell in region 6), and then column A would be left with no open cell for its marker. So D1 can't be the marker there.
6. From there, if E1 were the marker, row 2 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 F1 it would force A3 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 D3 (the last open cell in row 3), and column C would be left with no open cell for its marker.
8. First, place a marker at C1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column C, and its region. Placing here also clears its 1 touching neighbour.
9. Next, place a marker at B4. Region has exactly one open cell left. Placing here clears the rest of row 4, column B, and its region. Placing here also clears its 1 touching neighbour.
10. 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.
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 remaining candidate in this region touches F3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
13. From there, place a marker at D3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column D, and its region. Placing here also clears its 1 touching neighbour.
14. Following that, 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.
15. Finally, place a marker at E5. Region has exactly one open cell left.

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