This guide is a walkthrough of a main quest in the Witcher 3: Hearts of Stone DLC. The guide is composed of a map, map legend, and walkthrough to show readers exactly where to go and what to do to complete the quest.
Walkthrough
For this quest you’ll need to deliver 
Maximilian Borsodi’s House to Olgierd von Everec. Gaunter O’Dimm, your ever-helpful associate, will let you know that Horst Borsodi has an auction house in Oxenfurt, and he’ll recommend that you start there. You’ll find the auction house on the northern side of the city (#1), but when you approach it the guards will ask you for an invitation, which you won’t have. Luckily, right then Vimme Vivaldi will show up, and he’ll get you in.
Inside the auction house, Vivaldi will tell you that he’s planning on buying some paintings during the auction, and he’ll agree to introduce you to Horst afterwards. This conversation will earn you 100 xp. Vivaldi will then head upstairs to the gallery, and you’ll need to follow him.
In the gallery, Vivaldi will introduce you to three of his acquaintances. Talking to them isn’t required, but the conversations will earn you useful rewards. The three acquaintances are:
- Countess Mignole. She’ll sell you the diagrams for
Viper Armor, 
Viper Boots, 
Viper Gauntlets, and 
Viper Trousers. She’ll also lure you onto the balcony so she can ask you about Vesemir, with whom she had a tryst when she was young. You’ll earn 100 xp for talking to Mignole.
- Hilbert. He’ll play gwent against you for up to 250 crowns. If you beat him, then you’ll win the card Gaunter O’Dimm (siege 2 muster). It won’t make any difference if you give Vivaldi a 10% finder’s fee.
- Yaromir. He’ll ask you if you can identify a van der Knoob painting. The right answer is “the portrait of the merchant.” If you’re correct, then Yaromir will tell you how you can sell the van Rogh painting in the auction for a healthy profit. You’ll also earn 100 xp.
Note: Don’t tell Vivaldi about Yaromir’s tip, or else you’ll have to bid against him during the auction.
The auction will begin after you’ve spoken to Vivaldi’s acquaintances, or when you sit down on the bench in the back of the gallery. Vivaldi will offer you a loan. If you accept it, then Vivaldi will give you 100 crowns, but you’ll have to pay him back 110 crowns later on. Then three items will come up for sale:
Count Romilly’s Figurine. This will cost you 300 crowns. If you dismantle the figurine (at an armorer or blacksmith), then you’ll trigger the quest A Dark Legacy.
Starry Night over the Pontar. This will cost you 20 crowns, unless you told Vivaldi about Yaromir’s tip, and then it’ll cost you 60 crowns. If you received Yaromir’s tip, then purchasing the painting will trigger the quest Avid Collector. Regardless, you’ll also be able to use the painting in the Blood and Wine DLC.
The Professor’s Spectacles. These will cost you 350 crowns. You’ll be able to wear them (in a tool slot) to change the way you look.
After the auction, Vivaldi will introduce you to Horst Borsodi. Horst won’t like you much, and when you ask him about Maximilian’s house, his esteem will drop even further, and he’ll have his guards throw you out of the auction house. The guards will then try to beat you up, so you’ll have to box two of them at once. Luckily, the guards won’t be good boxers, and you should be able to beat them quickly just by repeatedly punching them.
After the battle, you’ll meet a hooded Stranger. He’ll want to become your “partner” in acquiring Maximilian’s house, and he’ll tell you that it can be found in the auction house’s vault. He’ll also suggest that he has a plan for its extraction, and that to help him out you’ll just need to visit the herbalist east of Oxenfurt and ask for “yarrow extract.” This conversation will earn you 200 xp.
When you reach the herbalist’s hut (#2), you’ll find three angry fallen knights outside trying to break down the door. After dispatching them, when you tell the herbalist the passphrase, he’ll lead you down into his cellar — where you’ll meet the Stranger again. This time the Stranger will be a little more forthcoming about his plans, and you’ll learn that you’ll need to recruit a burglar and a safecracker, plus “cull” some of the auction house’s guards. This will trigger three quests: Open Sesame: Breaking and Entering, Open Sesame: The Safecracker, and Open Sesame: Witcher Seasonings. You’ll earn 100 xp at the end of the conversation.
After completing the three quests, you’ll need to return to the herbalist’s cellar, where you’ll find everybody waiting for you. The Stranger will review his plan — the burglar will get you in, the safecracker will open the vault, and you’ll escape via a secret passage — and then he’ll tell you to show up at the auction house at midnight. This meeting will earn you 100 xp.
When you show up for the heist, everything will go according to plan — more or less. You’ll get inside safely, and you’ll find yourself in the attic wearing a “blue” version of New Moon armor, a King Foltest Mask, and a Wooden Sword. You’ll then need to make your way down to the basement where the vault is. However, along the way you’ll run into four guards. Even with the Wooden Sword (plus your signs), the battle shouldn’t be too tough. After defeating two of the guards, Horst will wander into the room, see what’s going on, and flee into the vault, locking himself inside.
During the commotion, somebody will raise the alarm, and guards will surround the auction house. The Stranger will take a hostage, and you’ll have to make a choice. If you override the Stranger and release the hostages, then the guards will attack you. If you keep the hostage, then you’ll need to make a reasonable set of demands to prevent the guards from rushing in anyway. Your four responses should be:
- “Pull back from the auction house.”
- “Bring us a wagon and horses.”
- “Doesn’t matter. Long as it rolls.”
- “I’ll set one free now. The rest later.”
No matter what you decide, Eveline the burglar will flee, and the safecracker will go down into the basement to open the vault door. If Casimir is the safecracker, then the explosion he causes will alert the guards, and they’ll storm the auction house no matter how good you were negotiating.
If you have to fight, then you’ll face a shield-bearing negotiator plus two regular guards. You should take down the regular guards first and then gang up on the negotiator. For best results, wait for the negotiator to attack the Stranger, and then attack the negotiator from behind. Doing this once or twice will probably finish him off. Once the guards are no longer a problem, you’ll earn 150 xp.
When you enter the vault, you’ll trigger a trap and fall into a pit with four level 32 arachnomorphs. Luckily, you’ll still have your regular silver sword with you, so the fight shouldn’t be too bad. When you kill the last arachnomorph, you’ll automatically climb back up into the vault.
Inside the vault, you’ll find lots of gems and jewelry to loot. When you eventually open the doors leading to the inner vault, you’ll finally encounter Horst, along with two more guards. During the ensuing conversation, you’ll learn that the Stranger is really Ewald Borsodi, Horst’s brother, and that the heist is all about a dispute over which brother should be controlling the auction house.
Horst will then try to bribe you away from Ewald. If Quinto is your safecracker, then he’ll join Horst, but Casimir will stick with Ewald. No matter what you decide, you’ll have to fight, but you’ll have your regular steel sword again, which will make things easier. When whichever brother you’re fighting against goes down, the battle will come to an end.
At the conclusion of the fight, the winning brother will kill the losing brother, and you’ll learn that Maximilian Borsodi’s House is the container that holds the family will. The winning brother won’t want to give it up, and this will give you two options. You can kill the winning brother and take the house by force (which will later cause Vimme Vivaldi to take over the auction house), or you can point out that you only need the container while the brother can keep the will, which will get you the house without a fight. For picking up the house, you’ll earn 100 xp.
Note: If you want to kill both brothers, then it’s easiest to fight Horst first and then Ewald.
To escape from the inner vault, you’ll need to exit via the secret passage. If you sided with Horst and he’s still alive, then he’ll open the way for you. Otherwise, you’ll need to search the vault using your witcher senses. You’ll find a secret door on the southern side of the chamber, and you’ll find a pressure plate on a column on the northern side. Pressing the pressure plate will open the door.
Note: Before leaving the inner vault, you might want to loot all of the containers. You’ll find more gems and jewelry, plus an Ofieri Crossbow and the diagram for Viper Venomous Steel Sword.
Past the secret door, you’ll find a tunnel leading to a house on the Oxenfurt docks (#3). Reaching the house will earn you 150 xp. Then to complete the quest, you’ll just need to take Maximilian Borsodi’s House to Olgierd in the Alchemy inn (#4). This will earn you 250 xp.
