You will receive this quest from Reyfald who is standing near The Gref's Rest (M1,2). He will ask you to steal a special drink - the Fennlan liquor. It is said to be hidden in the basement of the inn. You will require some thief skills to obtain it.
Go to the inn, and then walk downstairs. The place you're looking for is at the top of the room and it's protected by various traps. Select a character with the highest sneaking level and walk in stealth mode towards the wines. You should be able to notice the traps on your way. Try to disarm them all. They are on 4 and 5 difficulty level. The alcohol that interests you is in the group of bottles, first from the left. You can also open the chest, inside you will find a potion and 50 silver coins.
Now go back to Reyfald. You will receive 100 coins for the alcohol. Sadly, once he drinks it, he will start feeling bad. Go to the inn owner - Haeferic and ask him about the bottle that you have stolen. He will tell you that he has poisoned the drink on purpose. Sadly, preparing the antidote won't be easy. You must go to the russetwood and obtain the river reeds and ice troll's heart.
Ice troll's heart can be found in the western part of the russetwood (M2,3). There you will find three ice trolls accompanied by three lagufaeths. Be careful when fighting them as you can die very easily. It is a good idea to start with eliminating the less dangerous enemies and then focusing on the trolls. The herbal you're looking for can be found near the frozen lakes. The ingredient is located at the western shore of the pond in the middle of the map and near the small pond to the South-East. It can be also found at the north-eastern edge of the location.
Once you obtain both the items, go back to Haeferic (M1,2) and receive the antidote from him. Leave the building, talk to Reyfald and give him the medicine. This will increase your reputation in the city. Additionally, you will receive the Pilferer's Grip - gloves that increase dexterity and reduce the armor limits.
You can simply tell Haeferic that Reyfald wants to steal his liquor. You will receive 200 coins from him, but the quest will end in failure.