Ok then, let's recap. The world of Sinisistar, or at least the country it takes place in, is afflicted by events called "Abomination Outbreaks", byproducts of entities from a different plane of reality finding a foothold in the Human World. These entities are nameless, formless, and even if they can never fully manifest in the world they can mess with the natural laws and reality; therefore, they have been given the name "Otherworlders". The giant eyeball with tentacles we call "The Observer" and the monster summoned on top of Arcezon Cathedral, which was named "The Father" by the deranged cultists, are both forms summoned to manifest these Otherworlders -but never their full selves.
So far there have been no "evil" Otherworlders, with even the successful manifestations taking a passive role of expanding their influence. However, they are willing to exert their influence on people with the potential to bring them into the Human World, which is why they pick priests who will eventually become the members of cults around their respective Otherworlder. The very nature of these beings is alien to humanity, not just in understanding their presence but also in the way they operate: killing or mutating humans, women included, is simply a normal thing to do when they enter the Otherworlder's domain, and those women who prove to be worthy vessels are broken so they serve this purpose.
Abomination Outbreaks are byproducts of the influence of an Otherworlder on the land growing stronger, changing the laws of birth and allowing unnatural monstrosities to become the norm. They can also happen by human intervention, with the cultists using the power they got from their master and their magical talents to transform sets of people or even entire villages. That said, the cults prefer to remain undiscovered and limit their operations to isolated villages in their area of operation, eventually leaving those places if there is nothing left to research, but sometimes a solitary priest remains to keep tabs on potential mothers arriving or even threats like the Sisterhood of Purification. The experiments that spread the outbreak are supposed to help in finding a method to bring their master fully into the world, but human understanding will ultimately fail to accomplish this. As for the smart monsters, cases like demi-humans have build their own religion based on stronger Abonimation Broodlings, but they also exhibit some form of devotion to the Otherworlder responsible for their creation.
Finally, the influence of an Otherworlder curses the laws of natural birth in the land and eventually makes it extremely rare that the previous species produce a next generation, including humans. Combining the experiments of the cults, the raids by demi-humans, the scarcity of food and medicine, and the lack of means to defend themselves effectively, human populations in the area grow thinner and eventually die out if there is no intervention from outside.