The Adversary is a powerful evil spirit from the X-Men universe and an ancestral opponent of Forge's people - he is a trickster who seeks nothing short of the annihilation of the universe via manipulation and grand games of skill and cunning.
The Adversary sought to destroy the universe and to create a new one in its stead. The Adversary was dissatisfied with the overall pattern of what he had made and unconcerned with the fate of individual threads, sought to reweave it again and again until he was pleased with it. Hence, the Adversary sought to destroy and to remake the universe and did not care how many billions upon billions of sentient beings would die in the process. The Cheyenne believed that all things were composed of energy; the Adversary intended to destroy the universe by burning all of that energy completely at once.
It is unclear whether he could truly create a universe or was older than the present universe, and whether or not he somehow had a role in the destruction of the previous universe and the creation of the current one. In his true form, the Adversary was not composed of physical matter. He could be fought successfully through magic but not though most forms of physical force. The exception was that the Adversary was known to be vulnerable to harm by cold iron, and hence to both steel and Adamantium.
The Cheyenne Indians had long known of the existence of the Adversary, and their shaman, the mutant known as Forge, was trained from birth to combat him. Forge's teacher was the older Cheyenne shaman Naze, who himself clashed with the Adversary. However, Forge rejected the life of a shaman and instead enlisted in the United States army and went to fight in the Vietnam War. During the war Forge's own command was wiped out by enemy soldiers. Anguished and enraged by the deaths of his men, Forge used the spirits of nine of his newly killed men to cast a spell that opened a mystical portal into another dimension, through which came demons that slew the enemy soldiers.
Years later, Forge became an inventor working for the United States government. Alien Dire Wraiths invaded his headquarters at Eagle Plaza in Dallas, Texas since he was working on weaponry to be used against them. Naze and various members and allies of the X-Men, including Storm, joined with Forge in battling the Wraiths. In the course of this Wraith attack Naze was killed and his form was duplicated by one of the Wraiths. That Wraith, in turn, fell under the mystical possession of the Adversary.
Many months later, the X-Man Storm, whose superhuman powers had been suppressed by one of Forge's inventions, went to Eagle Plaza to ask Forge to restore her powers. Forge was not there, but she instead encountered the Adversary, whom she believed to be Naze. The false Naze convinced Storm that Forge had fallen under the influence of the Adversary and was attempting to destroy the Earth. "Naze" and Storm went to the mountain in the Western United States that was the "heart of his magic". The false Naze had Storm go up the mountain to stop Forge, claiming that he could not go himself since Forge had set up mystical alarms and defenses against him (Indeed, Forge had no doubt sent up such alarms and defenses, not against Naze but against the Adversary himself.) Storm fought off the demons that attacked her, and then stabbed Forge, only to learn that he was actually attempting to close the dimensional gateway through which the demons were coming. In attempting to escape the demons, Forge and Storm both fell through the gateway and were transported to the Earth of a parallel dimension, on which apparently no other human beings existed. Thus the Adversary had rid himself of the two people he regarded as the only real threat to his plans to destroy and remake the universe. He offered them the chance to become the "world spirits" of the new Earth he planned to create and to be the parents of its new human race.
The Adversary had by now captured Roma, the goddess who acted as the guardian and caretaker of the known dimensions, and held her prisoner within her own Starlight Citadel. Forge restored Storm's ability to use her superhuman powers, and they escaped the world to which they had both been exiled, only to become prisoners of the Adversary in the Starlight Citadel.
On Forge's native Earth, the Adversary had jumbled elements of many different time periods at Eagle Plaza. The X-Men fought against the resulting chaos while their actions were televised across the world by reporter Neal Conan. The X-Men, their friend Madelyne Pryor, and Conan were all transported to another dimension, where they invaded the Starlight Citadel. Forge and Storm regained their freedom as the X-Men fought and distracted, the Adversary, who now abandoned Naze's form end revealed his true appearance. The X-Men and Pryor willingly agreed to sacrifice their lives so that Forge could cast the spell needed to close the Great Gate and defeat the Adversary. In casting the spell Forge transformed the X-Men and Pryor into beings of pure energy, which Forge gathered together and hurled at the Adversary, forcing him through a mystical portal which sealed shut behind him. According to Forge, the deaths of the X-Men and Pryor had assured that the Adversary would be sealed away for all time. Forge and Neal Conan then returned to Eagle Plaza on Earth. Conan's telecast of the X-Men's battle against the Adversary and their deaths were seen by viewers all over the entire word.
However, Roma then used her great powers to return the X-Men and Madelyne Pryor to life. Roma explained that the Adversary must not be thus confined forever, since through the chaos he brings could come positive change and growth. However, she said, the Adversary had indeed been "bound for an age" by Forge's spell.
Later, the Adversary returned again to our dimension due to the misguided antics of the villain Haven. The Adversary killed Haven upon his return, but was once again defeated by Forge, who was at that time leading the government sponsored mutant team X-Factor, after Forge reclaimed his mystic heritage.