Malefic

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958 - Ma'alefa'ak is born on Mars, his psionics warped.  
973 - Ma'alefa'ak crafts a genetic virus that corrupts the shared consciousness of the Green Martians, making them incapable of defending themselves from fire.  His crime is unprecedented, so he is placed in isolation on an outpost on Saturn.
8 years ago - Ma'alefa'ak discovers a crashed probe from the Brainiac worldship, learning the history of Mars, Earth, & J'onn J'onzz.
6 years ago - Ma'alefa'ak uses his repurposed technology to travel to Earth.  He activates a hidden Martian Worldforge in the North Pole & begins harvesting human DNA to rebuild his psionics, and to try to create a new Martian race.  J'onn J'onzz confronts him and is nearly killed before he is able to return Malefic to his prison on Saturn.
2 years ago - Ma'alefa'ak returns to earth, making it appear that J'onn J'onzz has become a murderer.  He is defeated by M'gann M'orzz and the entire Watchtower, and imprisoned in the Martian Worldforge.
1 years ago - Ma'alefa'ak is freed from his prison by Gorilla Grodd to bring him into the Legion of Doom.
Martian Manhunter holds such a bizarre place in DC history.  He's a beloved character and recognized as one of the most powerful superheroes in the world, but he's never maintained a huge solo presence outside the Justice League.  The result is that he doesn't have a lot of villains of his own.  Even his best nemesis, the White Martians, are really more an enemy of the League then him personally.  
The best exception to this is Malefic, a character designed for the express purpose of becoming J'onn's main nemesis.  He's not a character that's made a lot of appearances, but he is a very well conceived idea and something we could totally use more of.  

Malefic's Comic History

Ma'alefa'ak first appeared in John Ostrander's 1998 Martian Manhunter ongoing series.  Ostrander did a really fantastic job shoring up the sometimes strained mythology of J'onn J'onzz, and in the process he created Ma'alefa'ak.

In this story, Ma'alefa'ak was a Green Martian born without telepathy, cutting him off from his people's group mind.  His insanity led him to create the genetic fire plague that wiped out the Martian race.  He was also J'onn's brother.  The earliest arc of the series showed Ma'alefa'ak coming to earth, killing the people closest to J'onn's civilian identities and setting him up to take the fall, even turning the League itself against him by making them believe that he had, during his entire time on earth, been kidnapping humans and experimenting on them in an effort to repopulate Mars.  

The character has cropped up again here and there, most notably in the 2012 animated movie Justice League: Doom, where he became a member of Vandal Savage's Legion of Doom.  

Our Malefic Story

While Ma'alefa'ak is a super functional nemesis for J'onn, we needed to make a few minor changes to his story to make him work.  The idea of him being a Green Martian driven insane because he was born without telepathy totally works, as does him being responsible for the genetic weakness to fire that leaves Martians unable to protect themselves, We've all agreed that it works better for the White Maritans to be responsible for the death of the Green Martians, because it fits the story of Miss Martian better.  Because of this we've made a few small shifts to the overall timeline, making ​Ma'alefa'ak's transgressions happen before J'onn was ever born.

There's already a precedent for the Martians to have interacted with Saturn thanks to an older character named Jemm Son of Saturn, so we're just going to quickly adept that concept and say that Ma'alefa'ak was imprisoned there until he was able to escape to Earth.  Just having J'onn be the sole surviving Green Martian would be enough for Ma'alefa'ak to devote himself to destroying him without them needing to be brothers.   

Malefic's Costume

When Ma'alefa'ak appeared in the original Ostrander comic, he had a twisted, almost joker-like appearance.  You knew characters were actually Ma'alefa'ak in disguise when they shifted to reveal his terrifying smile.  He would also regularly be positioned up above his targets Predator-like, the spikes and chains of his costume giving him a chaotic silhouette.  Later, when he appeared in the animated movie Justice League: Doom he had a cleaner look but one that carried a lot of the same menace.  He had a cape that included a very high collar, covering his face, and still had the same chains slung across his chest.  

Obviously, this character is a shape-shifter.  His initial look can be more tattered and malevolent while later on, when he is freed again and brought to earth to join the Legion of Doom, His look with the collar is going to give him a stand-out appearance that will work incredibly well among the other villains opposing the new Watchtower.

Related Characters
~"I find it interesting that a lot of interpretations of this character remove the element of him being J'onn's brother - Young Justice made him M'Gann's brother, the Earth 27 fan-made universe is going to frame him as the Moriarty to J'onn's Sherlock Holmes, and now this one removed it to make Mal a truly evil ancient Martian criminal who just wants every other Martian dead. It's not a complaint or anything; I just find it to be a fascinating reoccurring pattern.I find it interesting that a lot of interpretations of this character remove the element of him being J'onn's brother - Young Justice made him M'Gann's brother, the Earth 27 fan-made universe is going to frame him as the Moriarty to J'onn's Sherlock Holmes, and now this one removed it to make Mal a truly evil ancient Martian criminal who just wants every other Martian dead. It's not a complaint or anything; I just find it to be a fascinating reoccurring pattern."
Very good observation!  You might find that whenever you're looking at a GOOD retelling of a comic book story (which we're at least attempting to create here), characters that were at one point connected by blood wind up losing those connections.  If I had to find a reason, it's because often, writers make characters biologically related as a way to make them feel more connected, but what they're really doing is just baking in forced connections that don't really feel organic to the story being told.  Maybe the best example I can think of is any story suggesting that the Joker might be related in some way to Bruce Wayne.  While yes, you've managed to complicate the lore and develop connections that feel immediately shocking... it doesn't really grow in any meaningful way out of the existing story and doesn't REALLY contribute to the story you've been telling up until now.

Malefic was introduced as J'onn's brother, but they were also the two surviving members of their species, so does that familial connection really connect them any more than they already were? It just feels like an extra, unnecessary element that is easily removed.  In our case, because we need to shift the timeline around because the White Martians, not Malefic, is responsible for the genocide of the Green Martians... well, this was an easy element for us to lose.  

Incidentally, thank you for the call out to Earth-27! It's easily the coolest fan-continuity online (although I swear, we hadn't discovered it yet when we started our own).  I've spoken a lot with Roy from Earth-27, and his model of comissioning several art pieces for every character is so ambitious it's rabidly becoming the single best source of fan art for obscure DC characters.