This is the next mission of the game's first chapter. You receive it immediately after winning the arm-wrestling duel that takes place during the "Where Rumors Gather" quest.
There is a commotion near the sewer by the tavern. Children are taunting an old beggar lying on the ground.
Reward: Medium Bag x1
Step outside the tavern. A few steps away, you should notice a beggar lying on a sewer grate. Approach him and give him 1 copper coin.
In exchange, the beggar will give you a mysterious Shabby Wooden Key without saying much about its purpose, then he'll walk away and disappear. This will complete the quest, and three new quests will appear in your journal.
Thankful to be shown such kindness, the old man handed over a wooden key, uttered a few enigmatic words, and suddenly disappeared.