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

This 6×6 board rates as hard (difficulty score 1016). 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 C3 and D3 and C5 and D5, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
3. Then, every open cell left in this region sits in row 6, so its marker has to land there; that clears every other open cell in the row.
4. After that, every open cell left in this region sits in column F, so its marker has to land there; that clears every other open cell in the column.
5. Now, every open cell in column E belongs to the same region, so that region's marker has to be somewhere in this column; that clears every other open cell in the region.
6. From there, every remaining candidate in this region touches B5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
7. Following that, if B1 were the marker, it would force A5 (the last open cell in region 4), then C6 (the last open cell in region 6), then D4 (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.
8. First, that one can't be the marker: at C1 it would force D4 (the last open cell in region 5), then E2 (the last open cell in region 2), then F5 (the last open cell in region 3), and column A would be left with no open cell for its marker.
9. Next, marking D1 would force C4 (the last open cell in region 5), then A5 (the last open cell in region 4), and then row 6 would be left with no open cell for its marker. So D1 can't be the marker there.
10. Then, place a marker at D4. Column 4 has exactly one open cell left. Placing here clears the rest of row 4, column D, and its region. Placing here also clears its 2 touching neighbours.
11. After that, 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.
12. Now, place a marker at B3. Column 2 has exactly one open cell left. Placing here clears the rest of row 3, column B, and its region. Placing here also clears its 1 touching neighbour.
13. From there, place a marker at A1. Region has exactly one open cell left. Placing here clears the rest of row 1, column A, and its region.
14. Following that, place a marker at E2. Region has exactly one open cell left. Placing here clears the rest of row 2, column E, and its region.
15. Finally, place a marker at F5. Region has exactly one open cell left.

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