Help a witcher buddy out with a tricky situation.
The Witcher 3: Following the Thread
This quest is found on notice boards in Novigrad. You need access to Skellige to complete Following the Thread, but you should definitely get it done before kicking off Act Two, as a key character becomes unavailable later in the game.
Visit the warehouse south of the Southern Gate and ask the guard about the Monster in the Bits contract (exhaust his optional dialogue for giggles). He tells you someone else is already on the job, and sends you on your way.
Move to the search area and look around with Witcher Senses to discover footprints, a corpse and bloodstains. Exit the hut and follow the trail until it enters the water. Swim across and search the platforms around the structure with the water wheel to pick up the trail again. Apply Vampire Oil and Black Blood, then enter.
Inside you find Lambert, one of your brother witchers, battling an Ekimmara. Help him dispatch it and loot it quickly - if you miss out, tell him you'll meet him back at the warehouse to give you the opportunity to collect a pretty rare mutagen.
Back at the warehouse you'll be sent in to see the supervisor, where Lambert reveals he's after more than payment for a contract. You can't prevent a fight breaking out. Afterwards, extract the information you need.
Your next target is in the Seven Cats Inn, which is pretty close by. When interrogating the elven assassin you can be polite and diplomatic to avoid bloodshed. Otherwise Lambert elects to kill her, which prompts three nearby elves to come to her defence, resulting in a bloodbath.
Lambert decides the two of you should split up for a bit. Your target is in Skellige. Fast travel to a location on Faroe if you can, or sail there via the closest signpost with a boat. In either case try to approach the western side of the island, as the east can be pretty dangerous. The easiest way to reach your goal is to find the village of Ringvald and take the southeastern path out of it, following the winding road around to the hilltop where your target awaits.
Hammond is not at home to Mr Reasonable so just beat him and his friends up. Loot his corpse to discover a quest item; read it to gain the clues you need. Head back to Novigrad to meet Lambert.
Save your game before you enter the Nowhere Inn where Lambert awaits you - you have a brief window here to challenge him to Gwent, which is a necessary step in the Gwent: Old Pals quest. Chat with Lambert to report what you know, but when he suggests going to visit Karadin, say you'll meet him there. Then quickly open another dialogue with him as he starts to stroll away, and play Gwent. Hooray! (note: I've heard unverified reports that you can also play Lambert at Kaer Morhen if you miss this chance, but I personally wouldn't gamble on it.)
At Karadin's house, Lambert asks you to do the talking. Exhaust Karadin's dialogue options to get a full picture of his circumstances and past. When the conversation concludes, you can let Lambert kill Karadin or tell him to leave it alone. There are no known consequences to this choice beyond Lambert being even rude than usual. In either case, the quest ends.