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non of the current metal vessel users died yet so there is no way to know right now. maybe after this arc if kouen or hakuryuu die, or a metal vessel user in sinbad's sidestory dies, then we could get an explanation on it.
now when it comes to my personal speculation. I think that the djinn would simply return to their dungeon if their user dies
We know that two djinns from Sinbad had a previous owner, so i suppose they come back to the dungeon
What Minette said. With the user dead, the "contract" between the Djinn and the metal vessel user becomes null, and the Djinn's dungeon can be made to appear again, although this is all speculation at this point.
Anyway.. We haven't seen any metal vessel users died so we can't confirm that the Djinn would return back to its own dungeon and wait for another suitable king to use its power for a very long time... But I think what Ladylola's statement could be a possible scenario. There could be a cycle to this
Djinns have to return to their dungeons, otherwise after 72 kings are chosen then boom the world is over, thanks for playing. The whole process of becoming a king has to involve conquering a dungeon, so passing a free djinn to a new user would be catastrophic without a method of earning it. How this can happen is a super good question though, I've always wondered it too ever since we heard about Scheherazade's first king.