The X-Men have faced a legion of enemies over the years. Most of them aren't exactly subtle, like the giant mutant-hunting robots called Sentinels or literal demons from hell. One foe on the opposite end of that spectrum is John Sublime.
Sublime is unique among X-Men villains; he's not a man, mutant, or machine. He's a sentient bacterium that possesses hosts. Unlike most of the X-Men foes, he has actually succeeded in destroying the team, in one world at least.
Sublime was introduced during Grant Morrison's run on New X-Men. His first appearance was in New X-Men Annual 2001, in a story by Morrison and Leinil Francis Yu.
Sublime was introduced as a standard-issue evil mastermind. He was also behind the U-Men, a group that followed the teachings of his book The Third Species like a religion. They cut themselves off from the outside world, which included wearing containment suits at all times, and gained powers by grafting mutant body parts onto themselves.
Part of his support for the U-Men included a secret organ smuggling operation in Hong Kong. That was where the X-Men discovered and freed the captive Xorn, who became a trusted member of the team and a teacher at the Xavier Institute.
In a shocking twist, Xorn was revealed to be Magneto in disguise. During his time at the X-Mansion, Magneto became addicted to the drug Kick. He also introduced the drug to mutant problem child Quentin Quire.
Kick was actually Sublime's bacterial body in a form that allowed him to possess mutants. Sublime gained control of Quire, causing him to start a riot at the Xavier Institute. It led to the deaths of two students, one of whom was the Stepford Cuckoo Sophie.
Magneto's actions under the influence of Kick were more devastating. After revealing himself to Professor Xavier, Magneto destroyed the Institute. After triumphing over his old rival, Magneto and his new Brotherhood of Evil Mutants headed to New York City. They tore a swathe through the Big Apple, killing over 5,000 people.
The X-Men were able to stop them at the cost of Jean Grey's life. It was Sublime's influence that caused Magneto to murder Grey. Her Phoenix Force powers were returning, making her the biggest threat to Sublime's plans. Sublime's access to Magneto wouldn't last, as Wolverine killed him to avenge Jean's death.
In the wake of Jean's death, Cyclops turned his back on the X-Men. In order to cope, an overwhelmed Beast took Kick, exposing him to Sublime's influence. This dark Beast would take over the world, creating a Days of Future Past style dystopian future. When he resurrected Grey with her full Phoenix powers, a ragtag team of X-Men opposed him.
The Sublime influenced Beast wiped out the remaining X-Men, including Wolverine. He was ultimately defeated by Grey, who used her power to separate Sublime from Beast. That left him vulnerable to attack from Apollyon, the last of the U-Men. After Beast's death, Grey repaired the timeline by making sure Cyclops didn't turn his back on the X-Men. In the process, he started a long term relationship with Emma Frost.
Sublime's influence lasted beyond the future Jean canceled. It went all the way back to World War II. Sublime was behind the Weapon Plus program, which created super-soldiers, beginning with Captain America. That included Wolverine, who was created during the 10th iteration of the program (aka Weapon X). Deadpool, Fantomex, Venom and Luke Cage have also had ties to the project.
Sublime was briefly an ally of the X-Men, specifically the all-female version of the team. He sought out their help to defeat his twin sister, Arkea. He had chemistry with Rachel Summers, but she couldn't trust him enough to start a relationship with him. It didn't help that Rachel knew Magneto killed her mother, either.
He aided the X-Men in killing Arkea but couldn't stand to be around them afterward, leaving the X-Men. That was Sublime's final appearance to date. It's hard to believe we've seen the last of him, given how effective he's been in the past.