The worshipers of blight
The Worshipers of blight, or simply The Worshipers, came into being as more gem cores were used in the war and began breaking open poisoning the world with blight. The church has three sections, one for each from of blight. The Worshiper’s of Blight have a complex belief system. Just as there are three known forms of blight, there are three branches of the church. Each branch worships a form of blight (Normal, mind, and corrupt) and attempts to find ‘true blight’ through its worship. Blight is seen as a god-thing with each type of blight being a different god-thing but all blight is seen as the same god-thing whose true form is yet to be revealed. They believe that impurities in our world prevent true blight from pouring forth and converting our world into a new paradise. Instead these ‘impurities’ cause the three forms of blight to come from gem cores and only partially convert the world giving it a poisoned dying appearance. While not evil initially they see non-believers and anything not immune to the effects of blight as the impurities in the way of their paradise. Once the bombs fell they became almost militant, seeing it as a sign that total conversion was near. Now they seek to convert or kill anything in the way of that. Using many creature formed by the blight and born of the apocalypse they play a major role in post-war civilization, one that most survivors take great strides to avoid. Despite being of the same overall religion the different branches of Worshipers often work against one another in an effort to prove their way is the true path. This server to help keep the zealots in check to a noticeable extent as the biggest threat to the church is itself. The primary faction, that of normal blight, is the most active. They regularly sent priests into settlements for recruitment and to scout for resources. The followers of mind and corrupt blight prefer kidnapping and forcing those caught into the church. Sacrifices to the blight and to make thrall are common as are curses that permanently warp the offspring of any female affected. They have no limits and no morality, entire settlements have been wiped off the map because they needed bodies.
Thrall is the lowest 'rank' these are mindless creations that serve as fodder for the Worshiper army. Acolytes are the lowest actual rank and are often sacrificed (though they don't know that) Priests are the second rank and make up most of the inner clergy. The High prophets of each form of blight are the highest rank. These beings are warped by blight and wield immense magic abilities.
Their goal is to spread their toxic blight as far as possible and turn the world into one inhabited purely by blight infected creatures. However each branch wishes their form of blight to rule so they often will counter the actions of their own church in attempts to gain the upper hand.
Numerous pre-bombs military depots have been looted giving them plenty of and a variety of weapons and armor. Elemental towers are their preferred fortification due to their low cost and high efficiency. Their churches dot the surface and the cave below Equestria. They lack a true stronghold but have numerous small hidden areas that are heavily fortified.
They arose part way through the war, gaining influence as the effects and potency of blight became more apparent. When the bombs fell they saw it as a divine event and became a shelter for the desperate.