In Sewer you can find a room with a boy in a cage. The only way to reach him is to use Teleportation.
The quest starts right after you enter the room. Get closer to the cage and talk with the boy. He has the same collar as the one that you wore in Fort Joy. He wants you to free him. Sadly, you can't open his cage.
However, you can do that by lighting torches that are at the ends of the star painted on the ground. You must light them all at the same time. Use, e.g. Fireball spell.
After that you learn that the boy is a mad Godwoken called The Mistake. You must face him and a group of Undead. Be careful because they often use an ability that inflicts Charm on someone in your party.
Killing Karon ends this quest.
Reward: 96,950 experience points