Out in the street, (if you're Neutural, you'll run into Yaevin, and you can either kill him or talk him out of a fight). Fight your way to a wounded soldier, who gives you a Key to the nearby door. Go downstairs and into the sewer, where you'll fight a monster. Exit to the northeast. After a brief conversation, head up the stairs and fight your way into the Cloister.
Enter the Cloister, kill the two guards, and loot a Key from one of their corpses. There is a Fireplace here if you wish to Meditate before the next phase. Enter the next room. This is all quite linear until you get to the end, with one final chance to Meditate and Save.
You'll fight Jaques de Aldersburg, then the King of the Wild Hunt arrives and gives you a choice. If you choose to hand Jaques over, the King will sort-of tell you a secret. If you choose to kill Jaques yourself, you'll get the same secret later, and you'll get to kill The King of the Wild Hunt.
When you return to "present day," Talk to Dandelion to trigger the final cinematic and the end credits. Congratulations on beating The Witcher!
last modified 21 June 2020