You missed the pack of thieves with the magic lockpicks which they ask you to deliver to one of their number who went to the serpent temple. The thieves are close to the crypt back in the Valeia area of the map. If you sold the lockpicks, threw them away, or sold them you are out of luck with the prisoner. If that's the case, console yourself with the fact that he only gives you information about the serpent temple, which you can (with some difficulty) figure out on your own.
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