At the end of Blood of Dawnwalker's prologue, you'll fight Brencis, the Knyaz of Vale Sangora, and quite obviously the game's main antagonist. It's one of those classic "you're not supposed to win this" boss fights, since losing to him plays a cutscene and continues the story.
But where there's a will, there's a way. Below I'll explain what happens if you beat Brencis, provide a video of fighting him, and the outcome. Be warned, this contains spoilers for some aspects of the game's ending sequence.
Can you beat Brencis in Blood of Dawnwalker's prologue?
The answer to this is a little complex—both yes and probably no. You can beat Brencis in your first fight with him provided you've learnt how to Perfect Block and dodge, though it will take a little while.
After you beat Brencis, a cutscene will play in which he effectively pulls you into hell and reveals his true form, Brencis, Grotesque Ascendant (also the final boss). Since this is still a boss fight, you theoretically should be able to beat it, but even after acquiring the strongest equipment I could on the first day, you do an incredibly small amount of damage to him, and one hit from him will invariably kill you, even if you upgrade your health in advance.
I can also confirm he gets harder in his second and third health phases, making it super challenging, and you can't quicksave, meaning you have to fight regular Brencis first for every attempt.
Basically, most players are supposed to die against this boss, as it lets you load a save when you do. Whereas if you die to regular Brencis, a cutscene plays, and the story continues, as if this is the intentional path. Presumably if you are able to defeat Brencis, Grotesque Ascendant, it lets you complete the game in the prologue, which is very funny, though we'll have to wait for someone to do it to confirm for sure.
If you want to give it a try, I recommend leveling damage (since any attack from the boss will kill you anyway) and using your vampire claws for their speed. Remember you can also press E to Shadow Step to the other side of Brencis's unblockable attacks and flurries.
You could also switch difficulty to 'Story' if you're really determined to see what happens when you beat his second phase, though be warned he is still very challenging.
