It's an additional quest that finishes the Dead Man's Party. Shani isn't happy, Geralt is trying to change her mood. Check the glossary and read information about Shani. To push the romance forward, bring her a rowan branch.
Mead and Rowanberry brandy will be enough as well, but in that case you must watch - when Geralt and Shani will drink the wine at the water, forbid Shani mixing alcohol, otherwise the date will end badly.
To lead the romance to a positive end, when Shani asks a question at the water and you can choose between not ending on a kiss or returning it, you must select the first option. Previous dialogue choices do not affect this point of the romance.
Once all is done, go back to Oxenfurt. You will find Olgierd in Alchemy Inn. You won't receive any payment from him, but you can report what you have learned. This will end the quest.