A Hard 11×11 Puzzle Solved with Forced Chain in 23 Steps

This 11×11 board rates as hard (difficulty score 1011). Solving it from scratch takes 23 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 row 1, 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 I2 and J2, 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 K4, 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 J1 would force K3 (the last open cell in region 5), then A2, B2, C2, and 1 more cleared (every open cell left in this region sits in row 2, so its marker has to land there), then H4 (the last open cell in region 3), and then region 6 would be left with no open cell for its marker. So J1 can't be the marker there.
5. Now, place a marker at I1. Region has exactly one open cell left. Placing here clears the rest of row 1, column I, and its region. Placing here also clears its 1 touching neighbour.
6. From there, together, two regions have open cells only in columns J and K; 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.
7. Following that, every open cell left in this region sits in column H, so its marker has to land there; that clears every other open cell in the column.
8. First, together, two regions have open cells only in rows 3 and 4; 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.
9. Next, 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. Placing here also clears its 2 touching neighbours.
10. Then, place a marker at H3. Region has exactly one open cell left. Placing here clears the rest of row 3, column H, and its region.
11. After that, place a marker at J4. Region has exactly one open cell left. Placing here clears the rest of row 4, column J, and its region.
12. Now, every open cell left in this region sits in row 2, so its marker has to land there; that clears every other open cell in the row.
13. From there, 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.
14. Following that, together, two regions have open cells only in rows 7 and 8; 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.
15. First, every open cell left in this region sits in row 9, so its marker has to land there; that clears every other open cell in the row.
16. Next, every open cell in column A 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.
17. Then, place a marker at B9. Column 2 has exactly one open cell left. Placing here clears the rest of row 9, column B, and its region. Placing here also clears its 1 touching neighbour.
18. After that, place a marker at A11. Region has exactly one open cell left. Placing here clears the rest of row 11, column A, and its region.
19. Now, place a marker at C7. Column 3 has exactly one open cell left. Placing here clears the rest of row 7, column C, and its region. Placing here also clears its 1 touching neighbour.
20. From there, 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.
21. Following that, place a marker at K8. Region has exactly one open cell left.
22. First, place a marker at F10. Region has exactly one open cell left. Placing here clears the rest of row 10, column F, and its region.
23. Finally, place a marker at D2. Region has exactly one open cell left.

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