“The line ends with me.”  That was what was scrawled in a shaky hand on the note with the ring.  The ring had been anonymously sent to the authorities in the night.  It was a very old silver ring, but evidently well cared for.  It had some kind of symbol on it.  Not knowing what else to do, they had a reknowned scholar come in to tell them what he could, in an effort to help find the owner.
But what he told them scared them.  This was the legendary Seal of Thieves, the mark of the King of Thieves and his Court.  It was said that no one ever tried too hard to catch him, for he was a good man.  But he had not been active in several years.  Some of the younger men doubted that he had ever existed.  But here was proof, and something very strange was going on.  Until further evidence was found, the ring was put into safe hands.

Jacques had been King of Thieves since he was twenty seven.  He’d had a lot of fun, and he’d even done some good.  But he was an old, old man now, and dying.  His beloved wife had preceeded him years ago, his children were all grown and had children (and in one case grandchildren) of thier own.  He was tired, and ready to pass on the title to the next thief.
But that was before the young man appeared.  He made it clear that he intended to be the next King of Thieves.  But Jacques had discovered that this young man was far from the kind of man to wear the Seal of Thieves.  He was interested only in the charmed life of the King of Thieves, and getting what he could for himself.  Quite the opposite of what had, for generations, been tacitly seen as the duty of the King or Queen of Thieves: the protection and well-being of the lowest classes, and the populace in general.  Jacques may have been ready to retire, but he was not willing to pass the title to him.
So he did the only thing he could think of; he broke the Line of Thieves.  The title could only be transferred if the Seal was stolen from the current living King or Queen.  Giving it did not give the title.  So he sent it to the authorities, and went into hiding.

Now, the young thief is ransacking the city for both Jacques and the Seal of Thieves, intending to force Jacques to reclaim the ring and then steal it back.

Possibilities:
The PCs come into town to be greeted by news of the string of queer break-ins in the guardhouses and the jewelers sector.  No one has any idea what’s going on, or why nothing was taken.  Maybe they meet the thief one night…

One of Jacques’ children or grandchildren, worried about him, is trying to find him, and the PCs look like they could help.

In an effort to figure out what the hell is going on, the authorities have offered a reward for information on the King of Thieves or his location.

Getting tired of searching for Jacques, the young thief could kidnap one of his relatives, trying to force him out of hiding.  Perhaps Jacques would need some help from, say, a band of tough young adventurers…

Maybe one of the PCs is destined to become the next King or Queen of Thieves.  Perhaps the young thief does manage to reclaim the Seal and force it upon Jacques.  The PC could either steal it from him before the other man has the chance, or steal it from the young man himself.