Unity

The earliest post-war mutants created by the F.E.V. were not only large, green, hulking mounds of muscle, but also extremely intelligent. These were the first Orcs and they were followers of a being that calls themselves the Master. It is not known if this Master is a man, a machine, the first orc, or simply a collective figment of the mutant's imaginations, but they say its voice calls to them, commanding them to reproduce and multiply. Since orcs are all sterile due to the F.E.V. the only way for them to create more of themselves is to kidnap other humanoids and inject them with the virus. This is a concept that the most intelligent among the Orcs refer to as 'Unity'. In their words, all will become as they are.
While this didn't start out as a religion, it quickly became one. Some wastelanders began to see the carnage wrought by the Orcs, hearing their devout message of Unity, and decided to willingly join the cause; some out of fear, others out of a desire to see the world burn a second time. While most were injected with the virus, the most charismatic among them remained as they were, so that they might spread the word of Unity throughout the wastes, a position that came to be called a Missionary. While most actually just stood on street corners and tried to convince people to join the Orcs willingly, many Missionaries took it upon themselves to lure unsuspecting victims back to their cloister.
It is not known how, but the Orcs who are part of Unity have access to high-grade military weaponry. The working theory is that the Master was a pre-war government figure, but nothing is known for sure. All who have attempted to find out have become a part of the Unity. In their various bases of operation, away from the work of the Missionaries, the Orcs are able to muster numbers large enough to challenge even the most fortified settlements. Once every three years, Orcs will march out of their hives and across the wasteland looking to expand their numbers even further. The sound generated by their machine guns, rocket launchers, and assault vehicles has earned the title of 'Storm March' by the wasteland's inhabitants.
Orcs are always a problem, but organized Orcs are a menace, and that's exactly what Unity is. As the original F.E.V. serum dwindled and new supply had to be found, the Orcs that were being created were coming out less intelligent than the first ones who had received the purest doses of the serum. The most intelligent among them guided the rest, but there were many who were too confused to work with the Unity that were banished to the wilds. Nowadays, the wasteland is full of Orcs that are nothing more than wandering hermits, unable to even speak.
The bottom line is that the leaders of most wasteland settlements are extremely worried, and for good reason; the Unity has the numbers, the resources, and the motive to wipe out the whole wasteland. Coordinated efforts by larger cities can fend them off, but it's not at all easy. It isn't uncommon to come across smaller settlements out in the open that are completely wiped out by Orcs in one night, becoming nothing but a ghost-town thereafter. The scourge of Unity is known far and wide to the wasteland and is only tolerated in rough and lawless parts of society.