A Hard 7×7 Puzzle Solved with Forced Chain in 16 Steps

This 7×7 board rates as hard (difficulty score 1064). Solving it from scratch takes 16 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Line confinement, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, every open cell in column G 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.
2. Next, every remaining candidate in this region touches B2, 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 B5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
4. After that, marking A1 would force B4 (the last open cell in region 6), and then region 4 would be left with no open cell for its marker. So A1 can't be the marker there.
5. Now, every remaining candidate in this region touches C1, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
6. From there, marking D1 would immediately leave region 1 with no open cell for its marker, so it can't be the marker there.
7. Following that, if E1 were the marker, it would force C2 (the last open cell in region 1), then A3 (the last open cell in region 4), and region 6 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 F1 it would force C2 (the last open cell in region 1), then A3 (the last open cell in region 4), and region 6 would be left with no open cell for its marker.
9. Next, every open cell in column F 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.
10. Then, place a marker at E2. Column 5 has exactly one open cell left. Placing here clears the rest of row 2, column E, and its region. Placing here also clears its 1 touching neighbour.
11. After that, place a marker at B1. Region has exactly one open cell left. Placing here clears the rest of row 1, column B, and its region.
12. Now, place a marker at F4. Region has exactly one open cell left. Placing here clears the rest of row 4, column F, and its region. Placing here also clears its 2 touching neighbours.
13. From there, place a marker at D7. Column 4 has exactly one open cell left. Placing here clears the rest of row 7, column D, and its region.
14. Following that, 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.
15. First, 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.
16. Finally, place a marker at C3. Region has exactly one open cell left.

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