At the end of Baldur's Gate 3's Nautiloid prologue, you have a chance to earn the sick-nasty Everburn Blade greatsword off one of those 'supposed-to-lose' RPG fights. A "friendly" Mind Flayer is squaring off against the beefy Commander Zhalk while you're supposed to rush the ship's controls and hightail it out of there. The fight ends as soon as you activate the controls, but you could stick around and fight instead.
The thing is, beating Zhalk is way easier than you might think, and earns you a sweet flaming greatsword that can see you through most of Baldur's Gate 3's first act. Don't feel too bad if you're already like, 10 hours in and missed out on the Everburn Blade though—there's a ton of great weapon choices in this game—but if you'd like to snag the Everburn Blade as well as the "Devil's In The Details" achievement for beating Zhalk, read on.
How to beat Commander Zhalk
You have 15 turns to reach the Nautiloid controls and escape the ship, which is also plenty of time to beat Zhalk. The thing that makes it possible is that, no matter what, Zhalk will always focus the Mind Flayer as long as it's still alive (at least on Balanced difficulty and below). That means you can just throw everything at Zhalk without worrying about him one-hit killing you with his Everburn Blade.
At the start of the fight, I like to have Shadowheart (you did rescue Shadowheart first, right?) buff the Flayer with Armor of Faith to help him tank Zhalk's hits. The Mind Flayer will be your main source of damage as well as tanking against Zhalk. He turns hostile as soon as Zhalk goes down, but we'll be escaping immediately after that instead of trying to fight it.
I like to clear out the imps and lesser Hellboar with ranged weapons and the Intellect Devourer Us (as long as you also saved him) while closing in on Zhalk. After buffing the Mind Flayer and maybe using an Inflict Wounds cast on Zhalk, position Shadowheart close to the Nautiloid controls for a quick escape after the big guy goes down—she isn't much use against him otherwise.
Two more imps and a Hellboar will spawn once any of your characters move past Zhalk, but like the others, they're easy to take out with ranged weapons and your brain friend. More troublesome are the two Cambions that spawn back in the entrance hallway around when the counter reaches 10 turns remaining. They can usually reach Zhalk and the Mind Flayer's position around 7 turns remaining on the counter, and can one-hit-kill squishier characters (they do not share Zhalk's tunnel vision focus on the Mind Flayer).
Don't worry if anyone in your party gets KO'd—they'll be revived if just one person reaches the Nautiloid's controls. Don't forget to loot the Everburn Blade of Zhalk once he goes down.
Baldur's Gate 3 Everburn Blade stats
While the Everburn Blade doesn't get the classic +1 to attack and damage rolls you usually find on early game D&D weapons, its extra +2d4 fire damage is really impactful in Baldur's Gate 3's early game. I used this for a good few hours of Act 1 on my Paladin, but it also slots in nicely with Lae'zel or Karlach. Curiously, you don't seem able to sheathe the weapon—characters will just carry it unsheathed, presumably due to its fiery particle effect.