Stealing the Kutternberg Brotherhood’s Sword is an objective in the side quest Ars Dimicatoria in Kingdom Come Deliverance 2.
To start your journey to steal the Brotherhood’s sword, you must first complete Main Quest 14: Speak of the Devil.
You must find Menhard van Frankfurt and agree to fight him to begin this main quest. However, where you need to meet Menhard depends on what happened before the start of the quest.
If he is banished from the town, you will meet him at an inn west of Kuttenberg, or else you will find him on the corner north of Iron Eagle Tavern during the daytime while the remaining journey of the quest is the same.
How To Find The Guild Longsword?
Manhard wants you to head to the guild hall in the city’s southwestern corner and steal the brotherhood’s sword. He also wants you to hang the sword above the town hall.
He recommends stealing the sword at night time, which is the best option to avoid being seen.
You could get in through the front door, but it is a hard-locked door, so you might want to go through the yard to the right over some stacked logs to reach the hall’s backyard.

You still have to lockpick the door at the back or side of the building to get inside. Before approaching the door, you might want to put out the torch, as some people will sometimes walk through the yard and can easily spot you.
Also, you can drink lockpicking potion and Nighthawk potion for easy access and good vision during rainy nights.
Stealing Brotherhood Sword
After entering the hall, climb up the stairs to the first floor and then through the door on the right.
In the room, approach the head of the table, and you can steal the Guild longsword off the wall.
As Manhard said, you need to display the sword above the outer wall of the town hall.

Get out of the guild hall, fast-travel to Kuttenberg, hang up the sword, and go visit Manhard back at the inn.
You will now automatically go with him to the town hall. In the conversation, agree with him to confirm that you will help in their duels.