Open Sesame! - Witcher Seasonings The Witcher 3 Guide

In first order you should brew a potion

The quest starts after the first part of Open Sesame! quest ends. First you will receive a receipt for Cleansing Mixture. Three ingredients are needed to brew it:

• Dwarven Spirit - available at same herbalists, easy to find in random chests,
• Wolfsbane is a common plant, you can buy it at herbalists,
• Rotfiend Blood - as the name suggests, you must obtain it from the bodies of rotfiends (random drop). Many of them can be found in Velen, in the Farcorners (southern border), they are also often found on swamps and with groups of Drowners. Sometimes you can also encounter them in the Toderas village on the southeast.

Overhear the conversation of guards on the square

Brewing the potion doesn't end the quest. You must also deliver it to the cook. Go to the main square in Oxenfurt (M1,15) and use Witcher senses to overhear the conversation between guards. The first and second group won't have anything interesting to say (you must listen to the second group through the window). The last group will inform you that the cook is currently fishing.

The cook is fishing at the bridge

You will meet him at the bridge in the marked location. If you have Axii on level 3, you can use it to convince the cook to use the potion. Otherwise you must pay 500 crowns.