The first game had it's main themes as resisting temptation of both suicidal tendencies and pleasure. It is implied the whole game is the "observer" giving Rabiane visions of game over scenarios trying to figure out how to make the whole entering the world through humans thing work. The cultists weren't much use because they killed or Full Metal Alchemisted with random bugs and snails all the women in the region before considering the idea of birthing the outer beings into existence, and Rabiane was extra confusing because she self contradictingly craves motherhood and the sweet embrace of death. The endboss is the observer successfully tempting her with a snail into baby making sex after all that information collected then trying to put her into a sweet delusion. If she fails to break out too many times it's a game over with her giving birth to perfect replicas of the aliens the cultists worshiped.
The second game is a sequel with themes of horrors around parasites, impregantion with horrors beyond comprehension, temptations of lust, some horrors of transcendentalism, etc.
It's much lighter on guro because it's more about rape and monster babies. It does offer a very wide variety of REALLY random things though.
(Yes it's clearly a sequel. Not only Lelia is far more advanced than Rabiane in combat and range of magical abilities implying a long timeskip, several monsters literally created by humans in the first game also appear in evolved and adapted forms, AND the theme continues where the last game ended with pregnancy and giving birth to horrors.)