Venture into an ancient crypt to assist a stubbornly persistent researcher.
The Witcher 3: A Tome Entombed
There's quite a tough (optional) fight in this mission but the hardest part is finding the damn thing. Open your World Map and draw a mental line between the signposts for Heirarch Square and the Oxenfurt Gate. There's an entrance to the sewers just about on the halfway point. It's south and ever so slightly west of an innkeeper icon, and west and slightly north of two vendors. The entrance is a gate leading down into tunnels, not a manhole or ladder, and is most easily spotted at night thanks to a nearby brazier. I'm sorry. It's very hard to describe. You come here on a couple of missions, I think? But I forget which ones.
Assuming you find the entrance, follow the path around to the left and through the open grating. Heading downwards, go straight through to a brick tunnel curving to the left, and then start scanning the walls. You should spot a crumbling wall to the south east, which you can smack with Aard to reveal a second wall in need of knocking down.
Pass through and explore the crypt below. Eventually you'll come to a researcher examining a tomb. Apply Vampire Oil and equip Igni and Black Blood before proceeding.
Approach the researcher and he asks you to lift the lid of the tomb. Agree to help to reveal a cranky vampire. The lid is then closed. the researcher won't give up, though. Lift the lid again and the vampire loses his temper and attacks.
Succeed in beating up the vampire for a small reward and to conclude the quest. Don't forget to loot the corpse - Katakan mutagens are thin on the ground.