Tesham Mutna is the penultimate quest in The Witcher 3: Blood and Wine and is the final boss fight between Geralt, Regis, and Dettlaff. The decisions you made in the previous quest are vital for the fates of all involved, and it's time for Geralt to slay the Beast of Beauclair to stop the vampires from ransacking the city. This isn't going to be an easy fight, so you can find a guide on all three of Dettlaff's boss fight phases, along with tips on how to beat him.
All Tesham Mutna Outcomes
The fate of Syanna depends on what you did in What Lies Unseen or Beyond Hill and Dale:
If you completed What Lies Unseen, then you will be forced to fight Dettlaff.
If you completed Beyond Hill and Dale and got the ribbon, Syanna will live, and you will be forced to fight Dettlaff.
If you completed Beyond Hill and Dale and DIDN'T get the ribbon, Syanna will die, but you can choose whether to fight Dettlaff or let him go. Even if you fight Dettlaff, if Syanna is dead, you will be locked into the bad ending.
Before You Fight Dettlaff
Dettlaff is perhaps the hardest boss fight in the entire game, and has three phases. The boss fight begins with Regis and Dettlaff fighting in the distance, which allows you to prepare by any means necessary. Make a manual save before the fight, and don't bother with Vampire Oil, Axii, or Yrden, as these have no effect on Dettlaff.
The only thing I can truly recommend is Quen to shield yourself from his attacks, and White Raffard's Decotion, as it will instantly restore +35% Vitality, which is great for those on Death March who can't rely on a slow-healing food item.
When you're ready for the boss fight, approach Dettlaff and Regis, and good luck.
How to Beat Dettlaff
Phase One: Humanoid
Phase one is identical to the Beast of Beauclair fight from "Blood Run" mission. Dettlaff will swipe his claws forward, which opens him up for a fast attack before he dodges away. You should attack in intervals, waiting for his swipe to end so you can perform a quick attack yourself.
Dodging is better than parrying and blocking for this phase, so when he teleports or attacks, dodge behind him and try to land two fast attacks. Repeat this process, and make sure you have Quen activated at all times. Once his health lowers by one bar, phase two begins.
Phase Two: Flying
Dettlaff is at his most dangerous in phase two, and the main attack to watch for is when he begins to charge up in the sky with a swarm of bats. As soon as you see Dettlaff swarming with bats, run the other way and start spamming A/X to roll out of the way of them, as they can easily insta-kill you.
In order to attack Dettlaff, you need to be patient and either wait for him to crash into the ground, or force him to land by shooting him with a crossbow, Aard, or Igni while he's swooping down for an aerial attack. Land a few hits, like 3-4 fast attacks or 2 strong attacks, then dodge away and repeat until he has just one health bar left.
Phase Three: Dettlaff's Hearts
For the third and final phase, you will be pulled into an illusion of Dettlaff's heart. In each corner of the room, there will be three pulsing hearts, but they are impenetrable.You can only attack the hearts one at a time, and only when you've defeated the blood version of Dettlaff.
Be patient with the bloodied Dettlaff, and only attack after he's finished his three-hit combo. Land 2 strong attacks, then dodge and wait for him to strike. Once he dies, spam fast attacks on the heart to burst it, then repeat with the other two hearts for when Dettlaff respawns.
Once Dettlaff's final health bar depletes, the illusion will end, and Dettlaff will be stumbling in the arena. You just need to land one more hit on Dettlaff to kill him and end the boss fight, which begins one of these missions depending on the outcome of Syanna:
If Syanna lives, Pomp and Strange Circumstance begins.
If Syanna dies, Burlap in the New Stripe begins.
Should You Kill Dettlaff in The Witcher 3?
While what Syanna did was evil, Dettlaff is arguably worse. Dettlaff was a victim of Syanna's manipulation, but Dettlaff decided by himself that murdering others was essential to get what he wanted. Not only that, but Dettlaff is immensely powerful, highly volatile with emotions, and incredibly dangerous to anyone who isn't a vampire.
Dettlaff being killed by Geralt and Regis is the best ending, as it's clear that Dettlaff's emotional state got the better of him. This is a higher vampire who sent an army of lesser vampires to Beauclair, which resulted in hundreds, if not thousands, of deaths. Women and children were murdered by vampires, all because Dettlaff's heart ached for a woman, which made him kill four others.
Source: Game Rant