The X-Men's Adam-X, a dated remnant of the bad 'tudes and raditude that was '90s Marvel, just so happens to have the power to kill billions. The '90s were a strange time for comics. Pouches and gunfire peppered across the Big 2 and even the newly formed Image comics. Rob Liefeld strutted the Earth like a king (thankfully he didn't draw his own feet), he even had his own Levi's commercial directed by Academy Award-winner Spike Lee. Yes it was a weird and extremely profitable era of comics, and there's no better a reminder of the time than Adam-X.
Adam-X the X-Treme (yeah, that's his full title) is '90s down to his very core. He wears a backwards baseball cap and carries the aesthetic of a grunge musician along with the abilities granted to him as a Shi'ar/Mutant hybrid. A pair a claws akin to Wolverine's can extend out of his hands, and his main power is the ability to increase the temperature of blood to burn his enemies alive from the inside out. He certainly lives up to the cringe-inducing era in which he was created.
In X-Men Legends #2 from Fabian Nicieza and Brett Booth, Adam gets to cut loose with his powers, and it's pretty astonishing. Adam's blood manipulation powers allow him give people a sense of warm tingling to literal incineration, provided they have an open wound so the blood can oxygenate. It's a little bit complicated, but the X-Treme isn't limited by numbers. Meaning this silly bit of '90s edginess could kill countless numbers of people if the circumstances were right. Adam would make a fine addition to the list of Omega Level Mutants.
There are several applications to Adam's abilities that are outright terrifying. It's sort of a reverse version of the also-terrifying Iceman. Stick him on a modern day battlefield littered with everything from scrapes to bullet wounds and he could turn thousands into a pile of ash. X-Men Legends #2 suggests he could do the same to billions, which is wild even for comics.
It's honestly pretty hilarious that a character as silly as Adam-X is this powerful. The fact is, modern comics have often viewed Adam-X as a bit of a joke; an obscure, and embarrassing remnant of a time best left forgotten. This is like Austin Powers getting the powers of a god. Regardless of how goofy he may be, Adam-X would be an absolute asset for the X-Men.