The Arkham Knight -- the character, not necessarily the events of the video game -- is now an official part of the DC Universe, following Detective Comics #1000 and a story by Peter J. Tomasi and Doug Mahnke. Originally introduced in Batman: Arkham Knight, a video game from Rocksteady, that verison of the Arkham Knight was a mysterious figure who ultimately turned out to be Jason Todd. He appeared in the game as well as some tie-in comics, but Detective #1000 is the first time Arkham Knight has appeared in the main-line DC continuity. Who he is -- well, it's a mystery again -- one that will unfold over the course of the next arc of Detective Comics.
Like Action Comics #1000 last year, the final story in the book was written by the current series writer and set up a new villain to kickstart the next story for the hero. Ironically, when Brian Michael Bendis took over Superman and Action Comics, the former was vacated by Tomasi and Mahnke in order to make room. In the game, Jason Todd was kidnapped and taken to an abandoned wing of Arkham Asylum, where he was tortured by the Joker, who manipulated him into hating Batman over the course of a year. Joker faked Jason's death by sending a tape to Batman in which Robin was apparently shot and killed. He eventually escaped and began plotting his revenge on Batman for not saving and replacing him, taking on the name of the Arkham Knight. Eventually, Batman would get through to Todd, breaking through the brainwashing, and Jason would become The Red Hood -- a role he used until recently in the comics. Recent events in Red Hood and the Outlaws have put Jason out on his own as "The Outlaw," with a new costume and a darker approach, so it is possible (though probably unlikely) that the Arkham Knight of the comics could be Jason as well.
Whether Jason would actually blame Batman for his misfortune (which includes the death of his friends Roy Harper and Wally West at the start of the Heroes in Crisis miniseries) is anybody's guess, although it is worth noting that several characters in the recent The Flash/Batman crossover story certainly came very close to blaming Batman and The Flash for the events at Sanctuary. In any case, this Arkham Knight's philosophy seems to be that he is blaming Batman for...something. Tomasi's version of the character seems to be an inversion of the idea that Batman is "the hero Gotham deserves, but not the one it needs right now," insisting instead that the city deserves better than batman and that the Arkham Knight himself can deliver it.
The addition of a sword to the mostly-game-accurate costume could suggest another Bat-ally -- Azrael -- as the identity of the Arkham Knight, although again, that would require some explanation. The sword could also just have been added to go with the "Knight" motif, or could be one of a number of swords tied to Batman's backstory and mythology.
In the game, he was billed as the primary villain in the promotional materials, but it turned out that the challenge was not to kill or defeat him but to redeem him. No word on whether that concept will carry over into the comics. The mystery of his identity was a key driving factor in the marketing as well as the first bit of the game, so it would not be surprising for a similar mystery to drive the character's story in the comics.