Organization: The Forces of Chaos
Capabilities: The infinite armies of darkness borne from the minds of men. The Daemon, fragments of the Chaos Gods who take on form in the mortal world to spread the will and power of the Ruinous Powers. Reborn time and time again within the Warp, serving their Lords absolutely. And then there's the army of Chaos Space Marines, of which there are about 500.000. And they have tanks, nukes, titans, lasers, psykers, dreadnoughts, massive space ships and a pet goldfish. Of DOOM! They're backed up by Cultists and Heretics, as well as the armies of mutants formed from corrupted humans and humanoids throughout the Multiverse.
Membership Fee: YOUR SOUL!
Perks: Unholy satanic powers granted by the Ruinous Powers, should you win their favor. Should you prove an exceptional servant, they would grant immortality in the form of Daemonhood, elevating one to a Daemon Prince. Great dental.
Downsides: May eat you. Either because you failed, or because they feel like it. The Dark Gods are fickle that way. You also need to sit in the bus next to the Plague Marines.