Tony Stark is no stranger to making incredible, highly dangerous inventions on the fly, but nothing may top the time Iron Man combined a symbiote and his Extremis armor together to create his Extembiote suit. Even well after the King in Black event ended, the hero is still dealing with the repercussions of creating such a monstrosity - hinting that his time with the armor is far from over.
The King in Black's takeover of the Marvel Universe left Earth's Mightiest Heroes desperate to find a solution to stop the ancient supervillain. The God of Symbiotes uses his symbiote army to cover the entire planet in darkness while unleashing his forces against Marvel's heroes. Knull's takeover nearly worked, as infected heroes with symbiotes and even killed the most powerful hero in the universe, ripping Sentry in half. To stop the King in Black, Iron Man decided to infect one of Knull's symbiote dragons with the Extremis virus.
In King in Black #3 by Donny Cates, Ryan Stegman, JP Mayer, Frank Martin, and VC's Clayton Cowles, the Avengers take on Knull head-on, as the heroes are no match for the cosmic god. After trying and failing to bring Eddie Brock back to life, the team struggles with how to proceed against such an overwhelming force of power. However, when all hope seems lost, Iron Man arrives riding one of the King in Black's symbiote dragons, as he's injected it with the Extremis virus, connecting him to the beast.
Iron Man then uses his symbiote dragon to fly and turn it into Celestial armor to fight Knull's symbiote. Finally, taking over the Celestial, Iron Man fights against Knull's Celestial in the most epic match of Rock-Em' Sock-Em' Robots ever.
Iron Man would use the Extrembiote armor to assist Eddie Brock (powered by the Engima Force) to help defeat the King in Black. But, the Extrembiote armor's story is still very much active in the Marvel Universe. In Extreme Carnage: Alpha #1, Tony reveals to Flash Thompson that he's kept his living suit of armor, as he intends to continue studying and hopes to take full advantage of it one day - which honestly, seems like a recipe for disaster.
Among the many impressively dangerous suits of armor Tony Stark has ever assembled, the Extrembiote armor has to be both the most dangerous and one of the most powerful. Considering Iron Man is still studying and tinkering with his living suit of armor, the Extrembiote armor will undoubtedly play a role in future Marvel Comics stories. But, unfortunately, Iron Man doesn't understand when enough is enough when pushing the boundaries of scientific discoveries.