A Hard 11×11 Puzzle Solved with Forced Chain in 21 Steps

This 11×11 board rates as hard (difficulty score 982). Solving it from scratch takes 21 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 K, so its marker has to land there; that clears every other open cell in the column.
2. Next, every remaining candidate in this region touches J2 and J3, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
3. Then, every remaining candidate in this region touches F10, 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 G2, H2, I2, and 6 more cleared (together, two regions have open cells only in rows 2 and 3), then I4 (the last open cell in region 3), then G7, J7, G8, and 7 more cleared (every open cell in column H belongs to the same region, so that region's marker has to be somewhere in this column), and the chain continues, and then region 7 would be left with no open cell for its marker. So A1 can't be the marker there.
5. Now, together, two regions have open cells only in rows 2 and 3; 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.
6. From there, every open cell left in this region sits in row 1, so its marker has to land there; that clears every other open cell in the row.
7. Following that, place a marker at I4. Region has exactly one open cell left. Placing here clears the rest of row 4, column I, and its region. Placing here also clears its 2 touching neighbours.
8. First, every open cell in column H 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, place a marker at J6. Column 10 has exactly one open cell left. Placing here clears the rest of row 6, column J, and its region.
10. Then, place a marker at G11. Column 7 has exactly one open cell left. Placing here clears the rest of row 11, column G, and its region. Placing here also clears its 1 touching neighbour.
11. After that, every open cell left in this region sits in row 5, so its marker has to land there; that clears every other open cell in the row.
12. Now, 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.
13. From there, place a marker at K3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column K, and its region.
14. Following that, every open cell left in this region sits in column C, so its marker has to land there; that clears every other open cell in the column.
15. First, every open cell in row 10 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.
16. Next, place a marker at F5. Column 6 has exactly one open cell left. Placing here clears the rest of row 5, column F, and its region.
17. Then, place a marker at E1. Column 5 has exactly one open cell left. Placing here clears the rest of row 1, column E, and its region. Placing here also clears its 1 touching neighbour.
18. After that, place a marker at B2. Region has exactly one open cell left. Placing here clears the rest of row 2, column B, and its region.
19. Now, place a marker at D10. Region has exactly one open cell left. Placing here also clears its 1 touching neighbour.
20. From there, 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.
21. Finally, place a marker at H9. Region has exactly one open cell left.

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