Following the Thread - Novigrad
Quest Giver: Novigrad Notice Boards
Other Requirements: Complete this quest before The Isle of Mists or it'll automatically fail
This quest is started by accepting the notice from any of the Notice Boards in Novigrad. It's important you finish this quest before The Isle of Mists quest, this quest as well as many others will automatically fail if not completed before The Isle of Mists. Additionally, the quest Flesh for Sale will automatically fail if you don't complete it before the second half of this quest.
First thing you have to do for this quest is to speak with the Novigrad Guard (pictured above) who is right near the Southern Gate of Novigrad. He'll update your quest and send you to a building south of you. Search the body inside the building and Geralt will deduce we're dealing with a vampire. Follow the footprints to the dock and then across the river.
In the building on the other side of the river you'll find an Ekimmara fighting Lambert (pictured below). It's a really easy fight, give him a hand and take it out. Be sure not to fast travel with him once the fight is done so you can loot the Ekimmara.
Return to the first place where you spoke to the Novigrad Guard and speak with Lambert for a scene. Go inside, there will be more scenes followed by a brief fight. When you regain control of Geralt your next objective will be to meet Lambert at the Seven Cats Inn.
Note: If you haven't already completed the Flesh for Sale Quest talking to Lambert at the Seven Cats Inn will fail/complete the quest and you will never be able to do it.
Upon speaking with Hammond at the Seven Cats Inn your quest will update and you'll be sent to Skellige. In the southeastern portion of Skellige there will be a town by the name of Trottheim, Hammond is nearby here. Sometimes it will show a quest marker in the middle of Trottheim for Hammond but this is false - he's actually found east of Trottheim by some stone ruins, as shown on the map below. The road runs right into these ruins, so if you're having trouble finding him just follow the road.
Meet Lambert back at the Inn in Novigrad after finding Hammond to continue the quest. There will be some dialogue after which you'll leave the Inn and find yourself at Jed Karadin's house. Here you'll have some more dialogue followed by the decision of how you want to handle this.
You can kill Karadin, let Lambert handle the situation the way he wants or spare Karadin. If you read the note that you found near Hammond then you'll know Karadin was at least sincere in trying to give up his old way of life, seeing how he cut all ties. The rest of what Karadin says is left open ended, up to you to believe him or not.
Killing or sparing Karadin completes the quest.