Witcher Seasonings - The Witcher 3 DLC Hearts of Stone Guide
Before Begin with this quest you need: two rotfiend blood, two wolfsbanes, and a Dwarven spirit, now head to the marker in Oxenfurt. Make sure you don't draw any attention to yourself or get much closer. For this use your Witcher Sense to listen in on the conversation and repeat it three times before you obtain the information you need.
Now you have the information on cook's whereabouts, during daylight hours go to the pier and talk to him. Now you can either use Axii to add some of the mixtures to the food or pay him 500 gold to do it. Just return to the herbalist's hut and set the plan in motion once the team is assembled and the cook is on board.
Note: We had recruited Quinto for this Mission, so it may vary with Casmir.
Make your way down, as your inside the Tower and also take down the few guards which may ambush in the start. Now as the door will open you'll fall into a false trap which will lead you to face against Arachnomorphs. He is not that hard to take down, once you're done with him start looting before head out from the large doors. You'll also have the option to change sides.
If you Choose to Stick with Ewald:
This will end up fighting Horst, his two guards, and Quinto. After the fight is over, Ewald will refuse to hand over the house, this will lead to fighting him and once done you can take the house and its contents.
If you Switch allegiance:
You'll End up fighting Ewald. Horst will let you take the house, but not the contents. You'll also need switch on one of the large stone pillars to shut the doors and avoid the guards entering.
The Option is fully yours how you want to take it, but make sure to loot the vault and return to Olgierd to finish the quest and progress by learning about the third wish.