Warning! Spoilers for King in Black #2 below.
The King in Black has only just arrived on Earth and already the ancient God of Symbiotes has upended the Marvel Universe and shrouded the entire planet in darkness. As his symbiote dragons and his infected heroes wreak havoc on the planet, Tony Stark has figured out a way to seemingly stop the giant monsters - and it involves Iron Man's Extremis armor.
Marvel's Extremis arc by Warren Ellis and Adi Granov, saw Tony Stark injected himself with the Extremis serum after a near-death experience. The serum made Stark able to store nanotech from the suit within his bones and gave him an internal connection to wireless communications. Extremis also granted him powers that including a boosted brain function as well as a regenerative healing ability. Extremis was featured in the Marvel Cinematic Universe's Iron Man 3, where the villainous A.I.M. scientist Aldrich Killian eventually used Extremis to grant him superhuman abilities, fire attacks, and the healing factor.
King in Black #2 by Donny Cates, Ryan Stegman, JP Mayer, Frank Martin, and VC's Clayton Cowles, shows the Avengers scrambling to find a solution to take down Knull following the near-death of Eddie Brock. In order to save Brock, Tony Stark decides he's going to take down a symbiote to give to him. His quest puts him face-to-face with one of Knull's powerful Symbiote dragons, where Tony says he needs to figure out a way to "rewrite its DNA, break it down, and rebuild it simultaneously," as he pulls out the Extremis serum and injects the beast.
Despite Stark being infected himself, the plan proves successful as the nanotech strips enough of the symbiote which allows Tony to bring it back to the hospital and put it on Eddy. However, when the transfer is made, the Symbiote doesn't take and Dylan Brock is forced to use his powers to stop it. Brock flatlines immediately after the dramatic attempt to save him.
It's really brilliant thinking by Tony to try to use Extremis to use it on a symbiote dragon in order to strip it of some of its own powers. However, considering how the bonding nearly took over Stark and was incredibly dangerous, it doesn't appear it's the ultimate solution in takedown Knull. But, it still was one of the Avenger's first successful attempts at taking down one of his minions. Who knows? Maybe Iron Man can further reverse-engineer Extremis, even more, and figure out a way to stop the symbiote army for good.