WARNING: The following article contains spoilers for Future State: Superman of Metropolis #1 by Sean Lewis, John Timms, Gabe Eltreb, and Dave Sharpe, on sale now.
With DC's Future State in full swing following the Darkest Knight's final defeat, fans are introduced to a future of the DC Universe filled with new and familiar faces and threats. The pages of Superman of Metropolis #1 showed how Jon Kent was a much different Superman from his father, doing things that Clark would never even consider doing by bottling the city of Metropolis in a desperate bid to protect its citizens.
But in the process, the very threat he was trying to save Metropolis from unleashed a new weapon capable of killing full-blooded Kryptonians like Supergirl and the original Superman, and it may be even more dangerous than pure Kryptonite.
After bottling Metropolis in a desperate bid to protect its citizens from the effects of Brain Cells control, Supergirl showed up and was understandably upset by Jon's incredibly rash decision to bottle the city he was sworn to protect. But she seemed far more aggressive than she normally would be. It was then that Brain Cells revealed he had released nanotechnology called nirodhium. The immediate effect is that Kara suddenly became extremely aggressive, attacking Jon in a fit of blind rage.
Jon was unable to break through to Kara while in the Nirodium direct presence, so he drew her out into the open, hoping distance and time would undo its effects. But it wasn't until Kara recognized a greater threat that she fully snapped out of it. But just how quickly it managed to overpower one of the world's strongest heroes shows how dangerous this new weapon is. By all accounts, Kara was helpless before its effects, not even registering that she was being influenced by an outside force.
But even more intimidating about this new Kryptonian weakness is just how sinister it truly is. Normally, something that could kill a Kryptonian would injure them, weakening them physically until death took hold. But nirodhium attacks them in a different way, assaulting the mind and quickly unhinging them. And it's so effective because this is not some form of mental domination, but physically attacking the brain to stoke its target's more aggressive feelings. Of course, this wouldn't be dangerous to a Kryptonian unless it killed them over time, which it would.
But there is a silver lining: Brain Cells is a massive Kryptonian fanboy and as such does not consider Jon too much of a threat due to his half-human nature. Because of this, Nirodhium was calibrated specifically to affect full-blooded Kryptonians. This means that Jon has a resistance, if not outright immunity to this new threat to his people. He was not affected by the nirodhium's mental aggression and seemed to think and fight with a clear head. So should Brain Cells ever feel the need to use the weapon again (which he most likely will) then Jon is the best hope of stopping whatever is coming next.