Warning! Spoilers ahead for Batman / Fortnite: Zero Point #3!
For some odd reason, Deathstroke has the ability to speak in the Fortnite world when Batman, along with everyone else trapped inside, can't. The villainous elite mercenary first appears in dramatic fashion by holding a gun to Catwoman's head at the very end of Batman/Fortnite: Zero Point #3 by Donald Mustard, Christos Gage, Reilly Brown, Nelson DeCastro, and John Kalisz.
Ever since Batman found himself in the Fortnite world, he came to the realization that he was trapped in a nightmarish loop where he would forget everything that happened to him in the previous rounds as soon as a new one began. And the only reason why he knew all of this is because he had the foresight to leave messages for himself that, luckily, stayed right where he'd left them as soon as a new round started. On top of all that, he can't speak and doesn't know who he is, just like everyone else.
The only thing Batman remembers from his life before Fortnite is how to fight, though he's been able to connect on some sort of metaphysical level with Catwoman (and, to a lesser degree, Harley Quinn). But upon Deathstroke's sudden appearance, it's made abundantly clear that the villain somehow transcends the rules of this Fortnite world, for he speaks: "Don't do anything stupid. No sudden moves...or she's dead." In this case, the "she" is Catwoman. It might be unfortunate that he's holding Catwoman hostage and worrisome that she's still in the Fortnite world since it was originally believed that she had escaped. But the more pressing issue is how he can speak. (She is more than capable of taking care of herself, as she's proven countless times.)
What's also interesting is that Deathstroke doesn't identify Batman or Catwoman by their names during his big reveal. Either this is because he doesn't remember them, therefore proving that the only advantage he has over Batman and everyone else is that he can speak, or he does but the series' creators felt like holding onto that important nugget for a little while longer to keep readers guessing.
Regardless, readers are left in this sea of ambiguity in regards to why Deathstroke can speak and if he retains his memory. Another question is why Deathstroke hasn't appeared before now. The series has provided ample opportunities for the mercenary to show himself and has even featured sprawling images of, what has been assumed to be, all of the Fortnite contenders fighting each other (and watching Batman battle against Snake Eyes). Deathstroke hasn't been in any of them. Or could it just be him once again proving his aptitude for stealth?
Another possibility is that Deathstroke is somehow a part of this whole Fortnite experiment. That could explain why he is holding Catwoman captive, as it implies he caught her while she was trying to escape. Or could Catwoman be with Deathstroke? Remember, it appeared as though Catwoman stabbed Batman before she "escaped," and she's been prone to betray Batman at the most inopportune times.