The Hellfire Club is a mutant terrorist organization. Like the Mutant Underground, it tries to fight against anti-mutant actions, however, they believe that humans and mutants will never be able to co-exist and favor using excessive force and aggressive actions.
The prescise date the Hellfire Club started is unknown, however, they are shown to have been in operation as far back as the 1950's and were founded by the Brotherhood. The club is lead by a council known as the Inner Circle. Two of their members, Andreas and Andrea Von Strucker were mutant siblings known as Fenris (the wolf). Together they were responsible for a series of terrorist attacks around the world, in places such as London and Rio. Eventually the clubs activities brought them into conflict with the X-Men and they apparently "disappeared" while running from them.
Currently, the organization is attempting to revive itself and have sent several agents to infiltrate positions of political power. Two of their agents, Phoebe and Sophie Frost were captured while on their respective assignments and their sister Esme only narrowly escaped. Eventually Esme, who had tricked the Mutant Underground into helping her, was able to free her sisters from captivity, along with a handful of other mutants. She has since convinced Polaris to leave the Mutant Underground and join the Club, as well as Andy Strucker, Sage, Fade and Mark as they were convinced that the Underground wouldn't do what was necessary to ensure their survival.