How to get this quest
Complete Impostor Refugees then speak to Desmond in the Boar Hut basement.
Investigating Arwin
Go speak to Liliana and report to Desmond. Your next objective is to get inside Arwin's study. There are two options for this.
Once inside Arwin's study, note that there is a missable Sweet Memories drug in one of his shelves. Once you find the documents you are searching for, talk to Desmond and wait for three day. But another objective needs to be cleared before being able to ask Desmond for his results.
A way to Arwin's cellar
Go to the bandit leader's shed in the southern forest of Runes. There are stairs directly behind the leader's location (or former location if you defeated him). Those stairs lead to an underground passage with two unique bandits named Barry and Darry. Beating those two bandits is hard without the following tools:
Once you beat the twins, you can examine their spot as the last step of the investigation.