Manga

In Book of the New Sun (it's a science fiction tetralogy, and one of the best things I've ever read) the protagonist encounters a creature called an Alzabo, which absorbs the consciousnesses and memories of people it's eaten. It seems to be unintelligent itself, but it can attract prey then through a sort of mimicry, by vocalizing the thoughts of those consciousnesses, and that seems to be the entirety of the point as far as the creature is concerned.

Alongside that, there are people who ritualistically imbibe an enzyme taken from the Alzabo along with the flesh of a dead person, and duplicate the effect - temporarily absorbing the consciousness of the dead person. And that gets into some complex existential territory that's significant to the story.

Probably because I was primed by having already seen the concept elsewhere, I took the innkeepers claim more or less at face value - as a sort of seeming myth that's rooted in some degree of truth.

That's undoubtedly where the impression that the maneaters are both cause and effect came from too. The innkeeper isn't the only one who's essentially cultivated a maneater, and I've just assumed that there's some underlying reason, and that that's it. And when I saw the rumor about immortality linked to them, I assumed that the notion of just essentially getting some of their flesh stitched onto ones body was a misconception, and there was something more profound and likely sinister going on - specifically, possibly, that the actual mechanism involved in that nominal immortality is being eaten and, as the innkeeper asserted, absorbed by a maneater, then somehow being extracted from the maneater (and possibly that last bit is still in the experimental stage, and that's tied in with Heaven, and most notably with the monstrous babies - they're failed experiments).

Granted that that's all quite speculative, that's the rough outline of what I think of as the cause end of the cycle.

The effect end is of course much more obvious - the human-appearing hiruko apparently (certainly in some cases - we just don't know if it's universal) become maneaters. But even with that, we have that weird bridge between death and life again, since they don't simply transform - they die as humans and are reborn as maneaters.

As far as Robin goes, we don't have enough information yet for any solid guesses, but I think it's notable that his big issue is the (apparent - we self-evidently can't take it at face value) death of his sister, and the whole thing with the maneaters and the hiruko and the kids at Heaven deals with these weird cycles of death and life. The simplest assumption is that he's trying to bring her back, but this story hasn't followed the simple path yet, so it could be about anything. I'm sure there's something there though.