Kraken Fish
Short answer: a finned fish leaves exactly two possibilities — the fish is real, or the fin is the shape — and the finned pages stop at the cells both branches kill, which are the ones the fin can see. A Kraken fish carries the fin branch further with an alternating chain. If that chain kills the same candidate the fish kills, the candidate is dead either way, however far from the fin it sits.
What a Kraken fish looks like
Start with the fin. A finned fish is a fish spoiled by one extra cell, and the repair is a case split: either no fin is true, in which case the fish's counting argument holds and the shape leaves every cover cell outside the base — or a fin is true. The finned pages take the intersection of those two branches, and that intersection is small: only the cover cells sharing a box with the fin. A Kraken fish widens it by following the fin's branch along a chain of strong and weak links until it reaches a cover cell the fin never sees. Both branches then kill that one candidate, so it goes.
Read the chain as a run of alternating claims: a dashed link says the two marks cannot both be true, a solid one that they cannot both be false. Marking that alternation is a note-taking aid on your pencil marks — not how form9 is played. The board is solved with shapes, and nothing here depends on the colours in the app. On this position the fin alone already strikes the triangle from r1c6, inside its own box: that is the finned X-Wing one tier down. Everything the chain adds is the strike at r9c5.
How to use a Kraken fish
- Find a finned fish and bank what it already gives. Look for one shape confined to two or three lines that would be a clean fish but for one or two stray cells in a single box. Take the ordinary finned elimination first — the cover cells inside the fin's box. That is free, and a Kraken is only worth building once it is spent.
- Assume the fin, and say what follows. The fin is the branch the fish cannot rule out, so make it the hypothesis: suppose that cell really is the shape. Every cell it sees loses that shape, and every cell it lives in loses its other candidates. Those are the first links of the chain.
- Alternate to a cover cell the fin cannot see. Keep going, alternating: from a mark that is false, a strong link forces its partner true; from a mark that is true, a weak link kills whatever it sees. Aim the chain at a cell of a cover line that lies outside the base — those are the cells the fish branch already kills. Six to eight marks is as far as this stays followable.
- Strike only where both branches agree. The candidate you land on must be one the fish would kill as well: in a cover line, outside the base. Then it dies whichever branch is true. Anything the chain kills that the fish does not is proved by nothing — it rests on an assumption you never discharged.
What is written above is the common Type 1 Kraken, where one target candidate is killed by the fish branch and by the fin's chain. There is a Type 2, which drops the fin entirely and instead runs a chain out of every base line to one shared target — harder to spot, and the same idea. Both are cousins of the forcing chains and ALS chains this tier covers elsewhere: a case split, followed one branch at a time, until every branch agrees.
Kraken vs a finned fish
They are the same pattern read to different depths. A finned fish is a Kraken with a chain of length zero — the fin's branch is followed exactly one step, to the cells the fin itself sees, which is why its eliminations are always trapped inside the fin's box. Every extra link buys another step away from that box, and buys it at the cost of an argument the reader has to hold in their head. Take the finned elimination first, always. Reach for the chain only when the position needs the cell that lies outside the box.
Find Kraken Fish in your own grid
Paste a puzzle and this page will look for Kraken Fish in it — the same pattern shown above, in your grid instead of ours. Nothing is uploaded; it runs in your browser.
Frequently asked questions
What is the difference between a Kraken fish and a finned fish?
Only how far the fin's branch is followed. Both start from "either the fish is real or a fin is true". A finned fish stops at the cells the fin can see directly, so its eliminations always land inside the fin's box. A Kraken keeps going along an alternating chain, so it can strike a cell on the far side of the grid — at the price of a much longer argument.
Why must the target be in a cover line and outside the base?
Because that is the half of the argument the chain does not prove. The chain only handles the branch where the fin is true. The other branch is the plain fish, and a plain fish kills exactly the cover cells that are not base cells. A candidate outside that set survives one branch, and one surviving branch is enough to make the whole deduction worthless.
Do I have to colour the chain in?
No — and form9 has nothing to colour. Chains are bookkeeping on your own pencil marks: ticks and crosses, or two labels, or nothing at all if you can hold the alternation in your head. The app is solved with shapes and its colour setting is optional accessibility reinforcement, off by default. The technique does not touch it.
Is a Kraken fish worth hunting for by hand?
Almost never as a first move, and that is the honest answer for the whole of this tier. Solvers find these because they can try thousands of chains a second; a person finds them by noticing a finned fish whose fin has an obvious consequence and pulling on it. If you are stuck and no pattern is left at all, the trial and error page is the move that always works, and the sudoku solver will name whatever this grid actually holds.