The first game takes place in a land called Kessaria (apparently a city in Turkey IRL) and the second game in Arcezon, plus the fact that both final bosses seek to be born. Now, accepting the premise of this game being the prequel and ignoring the different names for the time being, take the following scenario:
1. Abomination Broodlings are killing the population in the countryside either by eating, straight up murder and torture, or using them as breeders. The latter issue is bad enough that many, if not ANY, children conceived between humans end up in the conception and birth of an abomination broodling that repeats the process in its place of birth. Villages are either quickly wiped out or experience a slower death in a year or even months due to nonexistent human births.
2. Local armies are not available due to isolation or lack of human power; perhaps the big cities are keeping their armies away to protect themselves, and only an ignorant fool would go and try seizing the abandoned countryside. The poor villagers still get taxed because fuck them.
3. Hanya and Leila are sent to deal with an outbreak in this particular town and its countryside, and to find out what the hell is going on. They are not the first to arrive because there were clues about the local sisters trying to fight off the otherworlder, or because those sisters were the first response. The events of the game happen, and Leila and Hanya lose because of the last-ditch effort from the Otherworlder (the last slug).
4. Years pass, and the problem has spread further to the point that the Sisterhood is running low on resources, maybe they lost some of their best, like Leila and Hanya. The bunch of petrified nuns in Kessaria's Cathedral can be used as an example.
5. Rabiane would be a very competent warrior nun, but one with more limited resources at her disposal because some of the core magical teachings have been lost in the years since the ending of this game. She has to clean up both the mess created by her predecessors and the outbreak connected to a new Otherworlder.
