Something rotten… is the final task in a specific quest chain within the 
Kuttenberg region. It serves as a grim conclusion to your dealings with the bandit group you may have assisted previously.
How to Unlock Something Rotten
To begin this task, you must have completed the Arrow-Head task (Arrow-head) and The Magic Arrow side quest (
The Magic Arrow). Once those are finished, travel to 
Grund and speak with Innkeeper Grosch.
Note: The quest marker may not appear immediately. You typically need to wait about five in-game days after finishing The Magic Arrow for the camp to reset and for Grosch to offer the dialogue option "How goes it?"
If you assisted the bandits previously, Henry will recognize the group Grosch is complaining about.
Confronting the Bandits
Grosch is fed up with a gang led by a man named Eggman. They have been making a mess of his inn. He will provide you with a Strong Bane Potion and ask you to eliminate the leader.
The bandits are located at a point of interest marked as Enemy Camp, situated just to the northwest of Grund. Be warned: there are six bandits in total, and four of them are heavily armored. Eggman is the worst, but there is also a strong captain with him.
Method 1: Stealth and Poison
This is the recommended approach for players who are not yet geared for a high-intensity fight.
- Wait until nightfall and remove any heavy armor to lower your noise level.
- Approach the camp from the side of the Grund village. Use the bushes for cover and make sure you do not kill anybody.
- If they find a body before they eat breakfast, they will not eat from the pot and instead start searching around.
- Sneak to the center of the camp where the cooking pot sits over the fire. Interact with it to pour in the Strong Bane Poison provided by Grosch.
Retreat from the camp and skip time until the next morning.
Eggman and several others should die after eating breakfast. If the objective does not update, wait an additional hour or two.
Method 2: Combat
If you prefer a direct approach, ensure you are wearing your best plate armor and have plenty of healing supplies.
Engaging six armored bandits at once is extremely difficult and can lead to a quick death if you are surrounded. Try to lure them out one by one if possible.
A good idea is to lure one or two out and then run to the village, which is close. Local guards will automatically attack the bandits. On this image a random Grund soldier can be seen helping and attacking the bandits:
Take Eggman's Hat and Loot
Once Eggman is dead, whether by blade or poison, you must retrieve proof for the innkeeper.
Locate Eggman's corpse, then loot Eggman's Hat from his body.
While you are in the camp, be sure to loot the various chests and bodies, as these bandits carry significant gear and Groschen.
Completion and Rewards
That is mostly it for this short task. Return to Innkeeper Grosch in Grund and present the hat.
During the conversation, you can learn the history of the hat. It apparently belonged to a pilgrim whom Eggman tortured.
If you have a high enough Scholarship skill, you can pass a medium check to suggest that a mass be said in the pilgrim's honor. This will grant you an extra boost to your local reputation.
Rewards: 150 Groschen, increased reputation in Grund.
After the quest, you may notice Innkeeper Grosch wearing the hat as a trophy of your success.
This finishes the Something Rotten task in Kingdom Come: Deliverance 2.
Something Rotten FAQ
What must you finish first?
Arrow-Head and The Magic Arrow, then wait ~5 in-game days before talking to Grosch in Grund.
Best method for most players?
Night poison the pot with Grosch's Strong Bane. Do not kill anyone before breakfast.
Poison did not work?
They may have found a body first, or you need to wait longer after morning skip.
Hard combat tip?
Lure bandits toward Grund so village guards join the fight.
Extra reputation?
Medium Scholarship when turning in the hat to suggest a mass for the pilgrim.