On this page of Assassin's Creed Valhalla guide, you will find a walkthrough of The Stink-Brew side mission.
Rewards for completing the mission:
- 1200 XP.
- Ledecester Sewer Door Key.
In Ledecester, in the place marked in the screenshot above, you will meet a woman who requires Eivor to bring her some viper eggs.
To get to the woman and start the quest you have to drop down, right by the river, and then enter a dark tunnel (watch out, though, there are snakes inside!). The woman is at the end of the tunnel, behind the bars.
You can get the eggs from killed snakes. There are quite a few of them near the sewers - about 13, and Odin's Sight will help you locate them. You have to collect 9 eggs and then give it to the woman to complete the quest. As a reward, you will receive 1200 XP and the sewer key.
Not every killed snake will have an egg, so it is worth saving the game and then trying to get 9 eggs in one streak. If this doesn't work, load the game and try again.
You can immediately use the sewer key. To get to the door it opens, you need to sneak to the next tunnel. You will find the hole on the way to the exit of the sewers.
There is one main chest behind the bards, with some iron ore, raw materials, and Silver in it. Watch out for one particular snake, which is placed in the middle of the passage. The second crate can be found on the wooden platform to the left, with additional raw materials and some Silver.