How to solve this 8×8 logic puzzle

8×8 · sharp · 17 steps

This 8×8 board rates as sharp (difficulty score 1000). Solving it from scratch takes 26 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Shape squeeze, Forced chain. Every step below is forced — nothing here is a guess.

First, place a marker at row 8, column 7. Region has exactly one open cell left. Placing here clears the rest of row 8, column 7, and its region. Placing here also clears its 2 touching neighbours.
Next, every open cell left in this region sits in column 1, so its marker has to land there — clearing every other open cell in the column.
Then, every open cell left in this region sits in column 8, so its marker has to land there — clearing every other open cell in the column.
After that, place a marker at row 2, column 6. Region has exactly one open cell left. Placing here clears the rest of row 2, column 6, and its region. Placing here also clears its 2 touching neighbours.
Now, marking row 1, column 3 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
From there, every open cell left in this region sits in column 2, so its marker has to land there — clearing every other open cell in the column.
Following that, every remaining candidate in this region touches row 6, column 4 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
First, marking row 1, column 4 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
Next, place a marker at row 1, column 2. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column 2, and its region.
Then, every open cell in column 4 belongs to the same region, so that region's marker has to be somewhere in this column — clearing every other open cell in the region.
After that, every remaining candidate in this region touches row 3, column 4 and row 4, column 4 — so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
Now, place a marker at row 3, column 3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column 3, and its region.
From there, place a marker at row 4, column 1. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column 1, and its region.
Following that, every remaining candidate in this region touches row 6, column 5 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
First, place a marker at row 7, column 5. Region has exactly one open cell left. Placing here clears the rest of row 7, column 5, and its region.
Next, place a marker at row 5, column 4. Region has exactly one open cell left. Placing here clears the rest of row 5, column 4, and its region.
Finally, place a marker at row 6, column 8. Region has exactly one open cell left.

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