A Hard 11×11 Puzzle Solved with Forced Chain in 19 Steps

This 11×11 board rates as hard (difficulty score 965). Solving it from scratch takes 19 logical steps, using 8 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Paired regions, 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 A, so its marker has to land there; that clears every other open cell in the column.
2. Next, together, two regions have open cells only in rows 1 and 2; 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.
3. Then, place a marker at B3. Region 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.
4. After that, together, two regions have open cells only in columns C and D; with one marker each, those two markers have to fill exactly those two columns, so every other region's open cells there can be cleared.
5. Now, place a marker at E4. Region has exactly one open cell left. Placing here clears the rest of row 4, column E, and its region. Placing here also clears its 1 touching neighbour.
6. 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.
7. Following that, 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.
8. First, every open cell in column K 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.
9. Next, every open cell in column J 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, together, two regions have open cells only in rows 10 and 11; 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.
11. After that, that one can't be the marker: place it at J5, and region 8 would be left with no open cell for its marker.
12. Now, every remaining candidate in this region touches K7, 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 J7. Row 7 has exactly one open cell left. Placing here clears the rest of row 7, column J, and its region. Placing here also clears its 3 touching neighbours.
14. Following that, place a marker at G5. Region has exactly one open cell left. Placing here clears the rest of row 5, column G, and its region.
15. First, place a marker at K9. Region has exactly one open cell left. Placing here clears the rest of row 9, column K, and its region.
16. Next, place a marker at A8. Region has exactly one open cell left. Placing here clears the rest of row 8, column A, and its region.
17. Then, 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.
18. After that, place a marker at I10. Region has exactly one open cell left. Placing here clears the rest of row 10, column I, and its region.
19. Finally, place a marker at D11. Region has exactly one open cell left.

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