If birthing the Otherworlder's vessel was this one time, "just impregnate her correctly" thing, the cultists would not be so willing to kill Lilia. They would be intensively trying to capture women, her included, alive.
But what are they doing?
They cornered the village with flesh eating zombies.
They fucking murder Lilia half the time in the northeast village. Set many lethal defenses in the cathedral, and you've got priests there who immediately try to chop her head off.
If it can get away with it, the Otherworlder just turns 2 potential mothers into purple meat pinatas when they land directly on its lap.
All that to say - they're clearly not in any rush. They're clearly not concerned about running out of mothers. And they don't seem to be trying to create the perfect, successful "baby vessel".
They're just churning broodlings and stillborns out at a steady pace.
That seems to be the plan. Quantity.
When enough Otherworlder flesh has been born into this world, it'll probably be able to merge it and inhabit it - unlike the invisible, half summoned avatar we fight.
It's a theory I said earlier. I'm pretty sure that to have a successful birth of the otherworlder, a mother needs to be particularly powerful. All the death traps could simply carry the logic of: if Lilia can't even get past this shit, she is clearly not worthy. In fact it very much seems like they only become worthy when their magic is at their peek, the purification ritual fully charged. If you lose the otherworlder fight early, you still only become a nanny to an abomination broodling. It seems like it takes imense magic to be able to properly host the otherworlder.
Also, it can also be simple information asymmetry. The minions of the Father don't know exactly what is needed to get him birthed into this world. That's why they took away pregnant woman to experiment on them. They are just throwing shit at the wall to see what sticks. Plus half of them went crazy anyway experimenting with other stuff, like expanding their minds that got them mind blown and turned into half corporeal brain slugs.
The minions of the Father are not a solid unit, they are not well coordinated and managed, they are a bunch of crazy fanatics and corrupted monsters fumbling over themselves each having their own stupid inaccurate interpretation of how to serve their master.
Even the demihumans for example are servants of the Father btw. Look at the masks the high ranking ones wear with the white face and central eye like design. Looks like the central slit on the face of the Father. However the deihumans probably don't even understand it, the worship the demihuman god cause they can see that clearly, and the father, being the master of the demihuman god, they can just kinda feel it in their soul, whispering in the back of their head. They get the urge to rape and impregnate but don't understand why. They make the woman they impregnated wear a mask and they paint the eye on the pregnant belly too. They are trying to please their unseen master, they just have absolutely no idea what the fuck they are doing and are clearly failing at their jobs.
The father probably can't properly communicate to his servants, his language way beyond mortal understanding. At best he can convey ideas or feelings to his servants, all the while his mere presence corrupts and drives them crazy.
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