A Hard 11×11 Puzzle Solved with Forced Chain in 20 Steps

This 11×11 board rates as hard (difficulty score 991). Solving it from scratch takes 20 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, every open cell left in this region sits in column B, so its marker has to land there; that clears every other open cell in the column.
3. Then, place a marker at C8. Region has exactly one open cell left. Placing here clears the rest of row 8, column C, and its region. Placing here also clears its 3 touching neighbours.
4. After that, place a marker at A7. Region has exactly one open cell left. Placing here clears the rest of row 7, column A, and its region. Placing here also clears its 1 touching neighbour.
5. Now, every open cell left in this region sits in column I, so its marker has to land there; that clears every other open cell in the column.
6. From there, together, two regions have open cells only in rows 5 and 6; 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.
7. Following 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.
8. First, place a marker at J2. Region has exactly one open cell left. Placing here clears the rest of row 2, column J, and its region. Placing here also clears its 1 touching neighbour.
9. Next, place a marker at D3. Region 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.
10. Then, place a marker at I9. Region has exactly one open cell left. Placing here clears the rest of row 9, column I, and its region. Placing here also clears its 1 touching neighbour.
11. After that, 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.
12. Now, 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.
13. From there, every remaining candidate in this region touches G5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
14. Following that, marking G4 would immediately leave row 5 with no open cell for its marker, so it can't be the marker there.
15. First, if H4 were the marker, it would force G6 (the last open cell in region 5), and row 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
16. Next, 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.
17. Then, place a marker at E6. Region has exactly one open cell left. Placing here clears the rest of row 6, column E, and its region.
18. After that, place a marker at H5. Region has exactly one open cell left.
19. Now, place a marker at G10. Region has exactly one open cell left. Placing here clears the rest of row 10, column G, and its region.
20. Finally, place a marker at K11. Region has exactly one open cell left.

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