Taking down the Justice League is pretty hard. Despite their best efforts, villains like Darkseid and the Joker have tried and failed more often than not. However, a minor DC villain did something that all of DC's far more impressive villains have failed to do: wipe out an entire Justice League team before it could complete its first mission.
In 1998's Starman #38, by James Robinson and Dusty Abell, Nash -- the gaseous Mist and archenemy of the '90s Starman -- slaughtered almost all of the La Fraternité de Justice et Liberté, the latest incarnation of the Justice League Europe.
Although Jack Knight is absent from this issue of his series, his absence may have saved him from a bloodbath that would've been at him in Game of Thrones. Instead, the Mist targets the array of minor heroes who've assembled for the inaugural meeting of this League. While she's posing as Icemaiden, she sits alongside Firestorm, the fiendish Blue Devil, the agile Crimson Fox and the adaptable Amazing Man.
For their first real mission, the team is given the task of protecting a jewel exhibit in Paris. Despite being a fairly mundane mission, the team grudgingly acknowledges that this is the type of job they are most suited to as superheroes, leaving a potential terrorist attack to the proper authorities while they do what they do best: prepare for a supervillain to try and steal the jewels.
At this point, the Justice League splits up to cover the museum. The separation allows for pairs of the League to do a little bonding. In a particularly incisive bit of foreshadowing, Blue Devil and Firestorm comment about how superheroes seem to be dying off more regularly now and Crimson Fox express a romantic interest in Amazing Man. But before Fox and Amazing can take their budding relationship to the next level, an emergency alert disrupts this otherwise uneventful guard detail.
The aforementioned terrorists managed to plant a nuclear in Paris, and the French government calls on Firestorm since his transmutational abilities are best suited to the crisis. Shortly after he leaves to deal with the situation. Crimson Fox finds a dead security guard and learns that the woman thought to be Ice Maiden reveals herself to be none other than Nash, the new Mist. And although Fox puts up a heroic front, Mist’s ability to slip by her targets with her foggy powers gives her the chance to subtly slit Crimson Fox's throat.
When Amazing Man finds Mist with the body of Crimson Fox, he's understandably infuriated. Using his powers, he absorbs the qualities of the wall of the room and prepares to give the murderous Mist a beatdown. Unfortunately for him, Mist has done her homework and covered the room with a veneer that made Amazing Man as fragile as glass.
After shattering him into a million little pieces, Blue Devil blasts Mist with his fire from his staff. However, Mist reveals that she has planned for this confrontation too. Devil’s fire powers trigger the sprinklers, which have been loaded with holy water. The water reduces the demonic Blue Devil to a skeleton who barely has the time or energy to warn a returning Firestorm about the bomb that's about to go off.
While Nash’s attempt to kill Firestorm failed, she still took out an entire Justice League before it ever completed its first mission. Although Blue Devil was eventually resurrected, these versions of Amazing Man and Crimson Fox both stayed dead. Although Nash never really let her attention stray from Starman too much after this, her actions and suggest that the Mist could've been one of DC's major villains if she really put her mind to it.