Bastion was "born" when the giant robotic Sentinel Master Mold, having absorbed the advanced Sentinel prototype Nimrod into its systems, fought the X-Men who saw no other way to stop the virtually indestructible amalgam except to force it through the Siege Perilous, a pan-dimensional portal which subjects those who pass through to the judgment of the universe's higher powers.
Re-emerging from the portal as an amnesiac Human/Sentinel hybrid with a seemingly immortal essence, he was taken in by a woman named Rose Gilberti. Living with Rose, she adopted him as a surrogate son and gave him the name Sebastion Gilberti. He began to hear about America's mutant problem. At some point, Bastion fell in with anti-mutant groups, like Graydon Creed's Friends of Humanity.
Bastion's prime directives were soon reawakened, and he began assembling the international anti-mutant strike force Operation: Zero Tolerance (OZT). They attempted to reconfigure the Sentinel force assembled by Project: Wideawake, but Bastion deemed most of them outdated. Instead, he had several humans secretly outfitted with nano-technology to be Prime Sentinels.
After Professor X voluntarily turned himself in after the events of Onslaught, Bastion confined him in an OZT facility along with the artificially-created mutant Mannites. Bastion also killed a Daily Bugle reporter who was going to report on Graydon Creed's history. He then captured Jubilee.
Soon after, Bastion put OZT into full effect, utilizing the Prime Sentinels and quickly capturing several X-Men members. Bastion took control of the Xavier Institute and accessed Xavier's files in search of the Xavier Protocols, despite opposition from Cable and Caliban of X-Force. The captured X-Men and Jubilee ultimately managed to escape. OZT was shut down by the U.S. government after Henry Peter Gyrich and Senator Robert Kelly asked the President of the United States to intervene.
Bastion was remanded into S.H.I.E.L.D. custody, but he escaped and returned to the former OZT facility. There, he absorbed the energy from the Master Mold unit with which he had created the Prime Sentinels and sought to transform Machine Man into the Sentinel Supreme. He was opposed by Cable, and seemingly destroyed.
Bastion survived and was recaptured by S.H.I.E.L.D. Death was sent to kill the Mannites, but also attacked Bastion, leaving only his head intact.
Bastion's head was recovered by the robotic Gatekeeper servants of the techno-organic being Mainspring. Bastion co-opted one of the Gatekeeper's bodies and used it to attack the techno-organic hero Warlock and his allies, being eventually defeated. Bastion's head was later recovered by Mainspring, who stripped away the layers of his programming to reveal the original Master Mold's source code and, using the alien transmode virus he was infected with, rewrote Bastion's programming, transforming him into Template. The virus soon took complete control of Template and forced him to construct a Babel Spire on Earth to signal the alien Technarchy. To oppose them, Mainspring destroyed himself along with Template, while Warlock managed to destroy the Babel Spire.
Bastion's head was later retrieved by U.S. government agents to access the information he had stolen from Xavier's files. The information was retrieved by Shadowcat, Wolverine, and Gambit, but not before Bastion attempted to sow mistrust among them by confronting them with misleading holographic scenarios. Although they got the files, the X-Men were showing signs of doubt within themselves and their team.
After the events of the Messiah Complex, the Purifiers infiltrated a S.H.I.E.L.D. facility with the help of some double agents and stole back Bastion's head and reattached it to a Nimrod's body. Once online, he alerted the Purifiers to the presence of the mutant team X-Force in their church. After their escape, Bastion realized there was no terrestrial force that could destroy the X-Men and finally eliminate the remaining mutant population, so he created one by capturing some of the prominent enemies of the X-Men and resurrecting others that had been dead. He took control of them by implanting the techno-organic virus of Magus on them. After that, he faced a coup d'�tat from Matthew Risman and his Choir, while also contending with the X-Force team which had followed Archangel, who sensed his stolen wings within the Choir. Bastion fought Wolverine, but calculated that the risk was too high to fight him directly and escaped with the resurrected X-Men enemies.
As part of Bastion and the Human Council's plan to kill the Messiah Child and destroy mutantkind, when Cable and Hope returned to the current time, Bastion ordered William Stryker's Purifiers, the Leper Queen's Sapien League, and Cameron Hodge's Right Smiley Faces to kill them. As a secondary plan, he ordered the death of all teleporters and transportation on the X-Men's Utopia base. A large dome appeared over the Bay Area to cut off the X-Men from escape or help. Once the dome was activated, Bastion opened a dimensional portal from his home reality to allow in more Nimrod-Series Sentinels. Like the original Nimrod, these Sentinels were able to detect, analyze and adapted to mutant powers. They were also self-repairing. The X-Men were barely able to subdue them, and several X-Men sustained severe injuries. Every five minutes a wave of five new Nimrod Sentinels continued to appear through the portal. Only by sending X-Force, Cable, and Cypher into the future to reprogram the signal and destroy the Nimrod Master Molds were the X-Men successful in defeating them. In order for the other members of X-Force to return to the current timeline, Cable was forced to go entirely techno-organic and held open the time portal sealing his own fate. Deciding that he would end the problem himself by killing Hope by his own hand, Bastion attacked. Hope, filled by the rage of losing Cable and the manifestation of her mutation, utterly destroyed Bastion and his dome in a single blast of tremendous energy.
Unbeknownst to everyone, Bastion had managed to avoid death by escaping into the future. Severely damaged and with his systems compromised by the time-travel, Bastion found himself in a future where mutants were still endangered, this time due to toxicity of the Terrigen Mists to them. His time on said future made him realize that he wouldn't have a purpose if mutantkind vanished from Earth, prompting him to decide to save them so he could continue doing his utility function.
Back in the present, Bastion reprogrammed a group of Sentinels to help him carry out his new mission, and they soon started to refer to themselves as mutants due to their new programming. When his Mutant Sentinels were confronted by the time-displaced original X-Men while aiding a new mutant who had emerged in Barcelona, Bastion had them captured and brought to his presence so he could try to convince them into joining his cause. However, the X-Men realized what Bastion was up to and made him reveal the truth, leading to a battle between the mutant team and the robots. Seeing the battle as pointless, Bastion fled with his Sentinels.
After Hydra took control of the United States of America under the leadership of a Cosmic Cube-altered Captain America, mutants were given part of Northern California so that they would not interfere with Hydra's new regime. Bastion started to work with Miss Sinister, Emma Frost, and Havok in order to use the Mothervine virus, a catalyst for artificial mutations, on a global scale. Following Mothervine's global release, Emma Frost turned against her allies after discovering the mutants generated by Mothervine could have their will subjugated. She liberated the group of young X-Men that carried out a failed assault against the masterminds behind Mothervine and attacked Havok and Bastion. The X-Man Xorn sacrificed himself and killed Bastion, swallowing him with the black hole inside his head.
