Oberon Sexton

History 

Origin

Oberon Sexton, nicknamed "The Gravedigger", is an English native.  He is the author of a book called "Masks of Evil".

He first appeared during a fundraiser party for Dick Grayson, hosted by Commissioner James Gordon.  He was later seen on the television show "Gotham This Morning, with Mo G", defending the actions of Batman and Robin.  The show stated that Oberon was in Gotham on the trail of a global serial killer.
 

Blackmail Victim

Oberon Sexton became the target of a blackmail scheme from an organization called "El Penitente".  Using a "secret" of Oberon's as leverage, they tried to force him to murder Batman.  Sexton met Batman later to discuss his alleged serial killer believing that the killer was targeting members of the Black Glove and that Bruce Wayne would be next on the killer's list.  Despite his blackmailer's demands, Sexton watched Batman leave.  For his failure to kill Batman "El Penitente" sent four assassins after him, but Oberon escaped his pursuers by a rope out of the window. 
 

Mysterious Ally

He went to Wayne Manor  where he aided  Damian Wayne  who was fighting several assassins.  Damian revealed that he knew Sexton was not really English, but was faking his accent.  He asked Sexton if he was really Bruce Wayne.  Sexton denied this saying he was worried that Wayne was the target of a serial killer.  However, he went on to confirm that he was not really who he seems.
 

Revelation

After the events at Wayne Manor, Dick Grayson met Oberon Sexton at his hotel.  Grayson brought up Sexton's identity saying that his partner, Damian Wayne thought Sexton to be his father, Bruce. Sexton again assures Grayson that he isn't Bruce Wayne, and Grayson seems to agree.  Grayson then brings up the Domino Killer, who Sexton had been investigating as well, and asks Sexton what the connection between all the murders were. Sexton points out that all the victims were members of Dr. Hurt's  Black Glove organization, but that's not what Grayson is talking about. Grayson, beginning to crack his knuckles, recites each of the murders and points out that each murder was committed with an old joke in mind. Realizing Grayson knows who he is, Sexton begins to take off his hat and glasses while Grayson continues. When he finally pulls off his mask, Sexton reveals himself to be not only the Domino Killer, but Batman's oldest foe, The Joker. 

Batman, with help from the Knight, found the real Oberon Sexton in his wife's grave in England. It was revealed the real Sexton had killed his own wife to commit the perfect crime. The Joker had killed him as "one last gag" and buried Sexton in his own wife's coffin before stealing his identity.