After something happened to the original Captain America, the government sought a new replacement to undergo the Super-Soldier process. While experimented on, they discovered that their choice was actually a mutant and the process had unleashed and magnified his latent mutant powers exponentially. Upon discovering his powers over vast psionic energies and being unable to control them, the mutant was rendered mentally unstable. Afterwards, the government claimed the mutant "Captain America-X" was killed by his own uncontrollable powers yet instead he lived and was allowed to become the new Captain America regardless.
Cap-X joined the Avengers and began a relationship with Black Widow, all the while keeping his mutant powers hidden from his teammates. During a battle, his powers began to trigger which made him flee in a bid to keep his secret. His fellow Avengers labelling him a coward, he was sharply ejected from the Avengers and Black Widow and Iron Man held a grudge ever since.
Captain America-X was created by Howard Mackie and Cary Nord and first appeared in Mutant X #15 although he was first alluded to in Mutant X #12.
Cap-X was initially introduced and it was uncertain and teased whether or not it was the real Captain America; Steve Rogers. After a while, it became clear that this wasn't Steve Rogers and they began teasing the fact he may be a mutant. Upon the confirmation he was a mutant, they then began teasing his powers which led to them finally being revealed in the start of the series finale and leading to a cataclysmic confrontation with his team-mates.
Major Story Arcs
After The Goblin Affair, the corrupt government-backed SHIELD began hunting down and executing mutants, Cap-X was charged with treason for aiding mutants in escape and for not betraying any mutants within the Avengers ranks.
Cap-X allied with a new iteration of the Hellfire Club consisting of himself, Sunfire, Sebastian Shaw, the Black Queen and Forge. Soon meeting up with The Six after the government nuked the X-Men's Mansion, the two teams sought to rebel against SHIELD and their forces. Unfortunately, Sunfire turned on the rest of the Hellfire Club and The Six after being blackmailed and threatened by SHIELD. The Hellfire Club went on their separate ways leaving Cap-X to join The Six as a new member, although he continued to keep the fact he was a mutant himself secret from his new teammates.
After saving Professor X from the Horsemen of Apocalypse, the Six were all then attacked by a telepathic attack by Professor X to immobilise them as he adopted his Onslaught persona and attempted to take over the minds of everyone on the planet. After battling Onslaught, the team discovered Professor X had allied himself with Mr. Sinister and the two had created their own "X-Man" who proved to be another tough match for the team.
After forming an uneasy alliance with Apocalypse and his Horsemen and saving the captive Scotty Summers from Mr. Sinister, the Six swooped in to battle Onslaught and Mr. Sinister. The team were each faced with having to fight psychic constructs of their greatest enemies and fears created by Onslaught. Cap-X was forced to fight a construct of the Red Skull who began hurling abuse at him for being a "mutant mongrel" leading to his team-mate's discovering his secret although the extent of his powers were not discovered. Onslaught and Mr. Sinister were eventually defeated and the planet was saved from mass telepathic intrusion.
The Secret Goblin Wars
After fighting alongside the team for a long period, on a mission to assist a rogue Canadian agent Wolverine escape from Canada, The Six were confronted by Cap-X's former partners the Avengers who wished to stymie a possible international incident between America and Canada. As the rest of The Six began fighting the Avengers, Cap-X accidentally unleashed a vast psionic blast which killed most of the Avengers as his sanity began to slip again he pummelled the surviving Iron Man to death with his barehands.
Cap-X continued his insane rampage against all those who he saw as "traitors to the American Dream", incinerating an entire platoon of Canadian soldiers and engaging in an intense battle with The Six's leader Havok. During the battle, the power unleashed by the two mutants inadvertently destroyed the moon, awoke the Beyonder and summoned the dormant Goblin Force. Captain America-X was eventually defeated and the Beyonder, who upon his awakening was chosen as the host for the Goblin Force, discovered the unconscious body and drained all his life force and memories, killing him.
Powers and Abilities
At full-power, Cap-X was an incredibly powerful mutant. He could create psionic-based blasts and shields and was able to incinerate a whole group of enhanced humans within seconds. His power when pitted against the power of Havok's, was so immense it was cosmically significant within the galaxy - destroying the moon as well as getting the attention of two higher entities. Cap-X could also telepathically read minds although his stability may have caused him to not directly realize this.
The super-soldier serum heightened his strength and stamina and he was able to demolish metal with his bare hands. He was a skilled hand-to-hand combatant and tactician and presumably had military training. Cap-X also carried a shield similar to the original Captain America's shield - although whether it was as indestructible was never seen.