This is the final showdown against Dettlaff. Geralt's master plan doesn't go according to plan. Dettlaff becomes enraged after Syanna vanishes (thanks to the ribbon you purchased from the flint seller), and quickly turns on Geralt.
The boss fight is in three phases. The first phase is pretty simple. Dodge Dettlaff's attacks, which makes him vulnerable. Strike with one or two heavy slashes and get out of the way. When he's lost enough health, he'll transform into a flying, eyeless true vampire. This one is pretty tricky. Even using the Qwen Sign won't fully protect you. His attacks take down a big chunk of health so dodge and roll often. He'll do a ground slam from above. That will leave him on ground level long enough for you to get a few shots in. If you take enough of his health away, it will trigger a cutscene. Dettlaff pounces on Geralt and bites him on the neck, transporting him into a bloody dream-like world. In that area, there are three sacks on the perimeter of the battleground. Attacking the sacks are the only way to drain Dettlaff's main healthbar. Watch out for the phantom Dettlaff stalking around the area. You can take the phantom down to get a brief reprieve, or just focus on primarily on the sacks. The best way to approach the battle is to attack a sack with a few hits at a time, and then move onto another sack. That makes the phantom chase you around rather than cornering you near just one sack.
Defeating Dettlaff completes the mission.