20,000 years ago Mars was inhabited by a race called the Burning. They were a vicious species that reveled in violence and fire, boasting a terrifying number of powers. The aliens reproduced asexually through a combination of psychic suffering and flames. The Burning were relatively primitive compared to the rest of the universe, but they were only centuries away from developing space technology.
At that point the Guardians of the Universe, who were observing the Burning and feared the potential destruction they would cause if they were able to travel to other worlds, personally traveled to Mars to neutralize the danger. The Guardians wouldn’t wipe out the Burning but they believed the species could turn into a powerful, peaceful civilization if their most violent tendencies were weeded out. The Oans genetically experimented on the entire species: they installed a psychosomatic fear of fire and split the Burning into two races. The White Martians retained the savagery of their ancestors while the Green Martians became a race of peaceful philosophers.
One of the Oan ships from that time crash-landed on Earth and was discovered by Vandar Adg. For two months the tribal leader fought the injured Burning member which had escaped. He finally succeeded in killing it in a sneak attack, but not before it had murdered his entire tribe of four thousand.
Trial By Fire
After the Justice League was trapped and killed in Obsidian Age Atlantis, J'onn J'onzz felt helpless and disappointed that he was defeated so easily. He blames himself for his friends' deaths because his fear of fire completely consumes him whenever he was exposed to it. He recruits the former villain Scorch to help him overcome his fear; in exchange, he would use his telepathy to help her deal with her own psychological problems. He succeeds but unwittingly breaks the genetic blocks placed by the Guardians in the process. No one knows when exactly the evil persona first surfaces.
Fernus itself doesn't show up during the first two issues of the "Trial By Fire" arc but makes its presence felt telepathically around the world. The world's criminals Inexplicably become remorseful of their actions; world leaders come together to make peace plans; extremist groups attempt to commit suicide for their actions; Lex Luthor is put into a catatonic state. Martian Manhunter undoes the mental tampering of Luthor at Fernus' displeasure, who psychically retaliates against the members involved and takes over J'onn's body. At the same time it mind-controls and attacks Ronnie Raymond, cutting him off from the Firestorm matrix and leaving him on the Moon's surface to die. While Superman and Wonder Woman travel around the world stopping the terrorist groups, Fernus searches unsuccessfully for Vandal Savage in Nepal. All of these actions are preparation for its spawning phase.
Fernus only reveals itself upon discovering that the League has Savage and the head of the Burning member he killed thousands of years ago. It lures them into the meeting room by leaving a near-catatonic Superman phased into the table. Fernus almost kills the League until Major Disaster's intervention allows them to escape to the Fortress of Solitude. The League tries to contact the White Lanterns for assistance, but Fernus anticipated that they would try this: when it incapacitated Superman, it stole the access codes from the Fortress and instructions on how to operate the Phantom Zone Projector from his mind.
Again he defeats the League and puts them in telepathic torment. This time a Guardian-programmed John Stewart teleports them out of harm. While John tells them about the origins of the Burning, Fernus distracts the League by hypnotizing mass numbers of people to attack their closest friends and colleagues. This allows it to launch nuclear missiles at major world cities in hopes of creating the perfect conditions for its spawning cycle. He manages to blow up the city of Chongjin in North Korea. In its ruins Fenris battles a newly determined Plastic Man (Batman's secret counter against Martian Manhunter) and part of the League physically while Manitou Raven leads the Atom and Major Disaster on the astral plane to contact J'onn's soul. With Scorch's help the League manages to put out the fires in the city and inspire J'onn to break Fernus' control. Fernus is ultimately atomized by J'onn's Martian vision.
Powers and Feats
Fernus possesses all the powers of the Martian Manhunter and his experience in using them: super strength and invulnerability, flight and super speed, Martian vision, telepathy, and absolute psionic control over its molecular structure (accounting for his shape-shifting, density control, accelerated healing, and ability to change size). However, these abilities seem to be larger in scope and slightly different in practical application. This may be due to its more ruthless nature.
Fernus controlled various groups of people around the world simultaneously, ranging from empathic control to straight-out telepathic domination. It was also capable of controlling massive groups of people through telepathic hypnosis. The second time it fought the League, it disabled them instantaneously through telepathy.
It has displayed incredible strength and durability as well. Fernus initially defeated the League (which consisted of Wonder Woman, Wally West, Ray Palmer, John Stewart, Faith, Batman, and Manitou Raven at the time) through sheer strength and invisibility. It also slaughtered roughly 500 White Martians in the Phantom Zone (soldiers with extensive training in tactics and controlling their powers) without effort, although it's likely that it preyed upon their weakness to fire. Fernus is durable enough to withstand the impact of a meteor sent by Major Disaster and the attacks of multiple League members without apparent harm. It was able to consciously operate and reconstruct itself from pieces.
Unlike J'onn, Fernus has powerful pyrokinesis and actually asexually reproduces in fire. It was powerful enough to put Scorch into a coma, who is usually immune to heat and flames.