Step 6 of 8
The last dark room
One room still dark. One cell that can light it. That’s the game, finished.
The rule of thumb: Endgame: one dark room left means exactly one cell can light it. Find that cell, place the torch, done.
The endgame has its own rhythm. When a single dark room remains, stop scanning the whole board: the torch goes on that room itself. Any other cell that could light it would sit in its row or column, unlit too, so it would be a second dark room. Once your sight lines are marked, the room is its own only candidate.
If the last dark room seems to have no candidates at all, something earlier is wrong — a torch shining where it shouldn’t, or an X on a cell the solution needs. That’s the moment to check the board over, not to guess. These practice boards all come down to this one room.
Where to look: Count the dark rooms before you count anything else. When the count is one, switch from scanning to tracing.
Worked example
A real position from a board that needs The last dark room. Purple: what the reasoning rests on. Dashed: the cells in question. Green: what it proves.
Endgame sorted. Two counting habits left, and they’re the ones that finish most boards. Step 7: Count a satisfied pillar →