Is a Novigrad side quest with suggested level 26.

Talking to the quest giver you can increase the amount he pays you buy at least 25.

The first two leads to hunting the monster are talking to the witness, and talking to the the medic who found the bodies.

Talking to the witness, she sounds like a drunk lunatic, and she gets mad.

Talking to the medic, he gets anxious about Geralt investigating the bodies. Examining the bodies, Geralt determines its a vampire.

Head to where the monster was seen, and investigate using your witcher senses. You'll discover a bracelet.

Head back to the tavern, and get drunk to bait the Katakan.

After getting drunk, Geralt will roam the streets singing drunk songs.

Eventually the Katakan emerges, and you will have to fight him, whilst drunk. After getting the katakan low, it will run away.

Follow the Katakan's trail. The trail will lead to a house, which you can enter via a ladder. Finish off the katakan, and loot the corpse.

Return to the quest giver for your reward.