Learn Swedish: Piraten med krokens kista saknas. - The pirate with the hook's chest is missing.

This is a phrasal genitive. The owner of the chest isn't just the pirate, but the entire descriptive phrase Piraten med kroken (the pirate with the hook). In Swedish, just like in English, the possessive -s attaches to the very last word of that entire noun phrase, making it krokens.

Wait, isn't kroken already in the definite form? Can we add an -s to that?

Yes, exactly! kroken is the definite form of krok (hook). When we need to make a definite noun possessive, we simply add the -s to the end of it, creating krokens (the hook's).

Why is it kista and not kistan? We are talking about a specific chest, right?

Even though it is a specific chest, Swedish grammar requires the possessed noun to be in its indefinite form (kista) whenever it comes immediately after a possessive -s. You can never use the definite form after a genitive, so krokens kistan is grammatically incorrect.

Why does the verb saknas end in an -s here?

The verb saknas translates to "is missing" or "is absent." It is a deponent verb, meaning it looks like a passive verb because of the -s ending, but it actually has an active meaning. It doesn't mean the chest is actively being missed by someone emotionally, but simply that the chest is gone.