Brencis is a formidable foe that will serve as Dawnwalker's final boss, but there are plenty of his henchmen that will serve as a thorn in your side along the way, and one of those is Ambrus.
This vampire lackey of Brencis is someone you can get the attention of by disrupting his blood transfer network. Leading to a much anticipated showdown between Coen and Brencis' loyal dog.
It's a tough fight, but one that can be made a lot easier if you don't mind doing a little extra legwork. Let's get into the nuts and bolts of this one.
How To Unlock This Quest

The Court Screen in The Blood of Dawnwalker Menus
To unlock this quest, the player needs to complete a series of side quests tied to Ambrus, which involve you killing soldiers at his camps, draining his blood drives, and generally serving as a pest he can't get rid of.
Doing 12 of the 15 possible Ambrus quests will be enough to gain his attention, and he will send word that he wishes to have a duel with you. Thus opening up the Guilded Gauntlet questline.
The Bittersweet Toast (Optional)

Erdene's Invitation to start A Bittersweet Toast
To begin this quest, you will need to head to Maragir, where you will come across a crucified towns-person with a note attached stating that until you agree to fight Ambrus to the death, more and more innocent people will die.
From here, you can wait until nightfall and head straight to the Ambrus fight if you choose. But you'll also receive contact from Iulian's servant, inviting you to come talk to Ambrus' right-hand man.
Provided you choose to do this, you'll learn that he is an unwilling servant of Ambrus and has been plotting his death for some time. He has a plan, and you're very much a part of it, as he intends to poison Ambrus through the blood of his personal blood cows. He just needs you to help with the poison.

A sit-down with Iulian
This will take you into the heart of the woods, where you'll seek Vera, a master poisoner. However, when you get to the hut, she's nowhere to be seen.
The next portion, for the sake of being succinct, sees you use focus mode to follow a path of dead animals, discarded traps, and clues, which eventually leads you to Vera at an Astral Communion Altar, dead.

Vera dead by the Astral Communion Altar
You are then given the information that Iulien is the blood cow Ambrus has been using, and the poison will kill him not long after Ambrus drinks his blood. So, you can choose to either help him or call it quits.
Assuming you choose to continue, you'll need to follow the path of blood, which will lead you to the cave of a lizard. It's purely optional, but you can then head inside and fight the lizard, gain some great loot, and even find a special ingredient that can help you save Iulien.

Fight with the Pale Ravager
If you're keen, enter the cave, and you'll begin a fight against the Pale Ravager. This beast can hurl poison spit at you, which deals damage over time, can burrow underground to avoid attacks, popping up to catch you off guard, and also hits hard.
This is a much easier fight during the night, as you can rely on your regeneration as a vampire. But it's still doable during the day provided you have health items and rely on some swordsmanship and witchcraft skills to deal damage without needing to be up close and personal.

Pale Ravager rewards
Defeating this beast will grant you various rewards, such as the Tatzelwurm Gland, Sunlight's Boon, and the Artisan's Longsword.
After this, return to the altar, and you can begin the ritual. You will need to wait until daytime to do this, but you can go away and do other things if you need to.
Beginning this ritual will see you lighting pillars and fighting waves of spectral enemies. Including a spectre mini-boss. This boss behaves somewhat similarly to Leonica from the Echoes of Silenced Bells quest. Albeit a little less deadly.

Rewards for defeating the Specters
The key here is to pick off the spectres early, as they will only get in the way, and this will then give you the peace to fight this mini-boss one-on-one without any cheap shots raining in from your periphery.
Defeating her will allow you to finish the ritual, and will net you the Slasher's Pendant into the bargain. This means preparing the brew. You can either prepare it as normal or include the Tatzelwurm Gland, which will ensure Iulian's survival.

Finishing the brew for Iulian
Whatever you choose, it's imperative that you have Iulian drink the entire brew; otherwise, he will die, and the boss fight with Ambrus will remain as tough as it would have been otherwise.
Completing this will end this side quest, and you will only have one night to make use of Iulian's poison. So, when night next falls, head to the duel spot of Ambrus' choosing, ring the bell, and begin the fight.
How To Beat Ambrus

Drinking Iulian's Blood before the Ambrus battle
The fight with Ambrus begins with a small chat of little consequence, and then the scene with him drinking the blood begins. If you didn't do the optional quest, Iulien won't show up, and he will drink blood from his servant instead, meaning he will be at full power.
Whereas if you do complete The Bittersweet Toast, he will first offer you Iulian's blood, and then drink himself. Be sure to pick an option such as 'I wouldn't trust anything you offer'. Drinking Iulian's blood will poison you too.

In battle with Ambrus
The effect of this poison means that, when you deplete one notch of his HP, he will not be able to use his special effect. This, in normal circumstances, is a timed period where any shots he lands on you replenish his health, and he gains invulnerability. However, when poisoned, this doesn't work, and he only gains a period of invulnerability.
As for the fight itself, Ambrus will use standard claw and sword attacks, and his very adept at blocking attacks, so you almost need to perform perfect blocks or sword arts to land hits on him.
But the real test is avoiding his unblockable attacks. One is a flurry of sword swipes, and the other is a charging attack. The charge is best dodged to the side, whereas the flurry of attacks is best avoided by stepping back and avoiding him completely while the flurry of attacks plays out.

Ambrus's final phase
The only other major development in this fight is during his final phase/notch, where he will glow red and spawn clones. These can be attacked and blocked, but it doesn't deal him any overall damage. So it's best to try to perform perfect dodges until the real Ambrus appears again.
From there, you should be able to whittle down his health and win the fight.
Rewards And Outcomes

Choosing to leave Ambrus, or invite him in
After the fight, you will be given the choice to drink his blood or simply kill him. The most beneficial option is to drink his blood, as this will raise your corruption and will also grant you the Scarlet Shield skill if you haven't got it already.
Plus, this will allow you to enter Ambrus' place of rest, where you will be given the option to let him rot or invite him inside, taking a small piece of him with you. Whether you grant mercy or penance is up to you. Just remember, though, if you do drink his blood, you'll still be poisoned.
Then, as for Iulian, if you choose to save him using the Tatzelwurm Gland, he will be most displeased and will hold it against you. So don't expect any sort of hero worship or reward.
Whereas if you do not, he will be dead after the fight, and you will have the option to bury him if you so choose.
As for rewards, alongside over 150x Denarius, you'll gain the Song of Valor and Ambrus's Cuirass.
The former is a very powerful weapon to use against mortal enemies with great activation charge perks. Then, the latter is a fantastic piece of armor that is best for your night build, as it offers vampire ability cooldown buffs, as well as activation charge-related buffs.