Little is known about the Eighth Brother. It’s said that he was once a member of the Jedi Order who was, like the rest of his fellow Inquisitors, turned into an assassin for the Galactic Empire. “Star Wars Rebels” Season 2, notably, features not only the character’s screen debut but also his apparent death. Indeed, shortly after it’s revealed in the series’ Season 2 finale that the Eighth Brother was sent to hunt down Darth Maul on Malachor, the Inquisitor’s lightsaber is irreparably damaged. When he then tries to use his lightsaber to fly away from his enemies, it malfunctions and he falls a great distance — presumably to his death.
However, the Eight Brother’s dead body is never actually shown at any point on “Star Wars Rebels,” which means it’s possible he somehow managed to survive the events of the show’s Season 2 finale. That, in turn, opens the door for him to be Marrok. It is also, alternatively, possible that the Eighth Brother did die as a result of his fall on Malachor but was subsequently revived by Morgan Elsbeth (Diana Lee Inosanto) using her Nightsister magic. That would explain why the character explodes in a cloud of Nightsister-esque green smoke after being fatally wounded by Ahsoka Tano on Seatos.
All of this is to say that it’s definitely possible that Marrok is either an older or revived version of the Eighth Brother. Fans won’t, of course, know whether that’s the case or not until “Ahsoka” creator Dave Filoni or someone else at Lucasfilm decides to comment on the matter. For now, though, of all of the fan theories that have cropped up about Marrok’s identity, this certainly seems like one of the most plausible.