The Order of the Cull was created after The Cull happened. They are a religious movement that intends on eradicating all the Nether in the world. They try to convert any survivor they can to their religion, even offering rewards for Nether parts, as proof that they killed the Nether. Nether parts can be traded for healing items, food, and even weapons.
The Order of the Cull believes that the Nether are just a test to mankind, and that the only way to end the apocalypse it to eradicate all the Nether.