Well, both are pretty much the given bad endings in the Kagura setting - some youkai "reproduce" by transforming human women into more instances of their species, while other youkai (some types of which are always male) reproduce by fucking human women. (At least as the surefire way to make more of their own kind. If youkai get it on with each other cross-species, I think the kid's from whichever species is "stronger.")
And youkai reproduction in this setting is actually magic, so they can impregnate a girl by fucking her in any hole at any time. It's closer to a curse/disease than a proper fertilization (the youkai fetus feeds on the victim's life energy), and a girl who doesn't get rescued and purified is pretty much doomed to pop out monsters for the rest of her life. Hilariously, human (or hanyou or kami) semen is the fastest way to neutralize a youkai contamination, which is why men are normally just killed by youkai, and also why there's always at least a male NPC present at the temple bases throughout the games. (The Reimeiki games cut out the purification scenes - no mundane human sex here! - but it's always been part of the full games.) Edit: And the whole "girls get fucked, men get killed" thing is part of the in-world justification for why it's always girls fighting the monsters - the girls with enough mana to become exorcists are prime breeding stock for the youkai, and so they're simply too valuable to kill out of hand, no matter how dangerous they are. So a girl who loses will almost certainly have a traumatic experience, but as long as she can be rescued by a partner or support, she'll be able to fight again. (Aside: In this setting, hanyous seem to come strictly from a man impregnating a youkai, under whatever circumstances.)
The bad endings in the series have generally been corruption endings (like in all of the full rogue-like games) or seedbed endings (like in the tactics games). The seedbed endings usually entail the party's home base falling to the villain and the girls getting claimed by said villain. Though in Tsukikagura it instead involves one of the girls being picked up and carried off by a western dragon to never be seen again, with a final cg scene showing the girl laying eggs or live-birthing half-dragon abominations. Heh. And in Ama Kagura, the girls are challenging youkai lords in their lairs with no hope of rescue if they lose (after all, your girl is the rescue party), so every stage has a bad ending scene for losing to the boss.
But to answer your original question... the plot synopsis from the product description indicates the threat is a massive and glowing monster (possibly a god of the land, like from Princess Mononoke), so I'm going to guess eternal imprisonment as a seedbed for the bad ending. If the text had hinted as something I knew was female as the boss I would've guessed corruption.