Why are cambions unable to reproduce amongst each other?

Question

There is an ancient, satanical practice that is performed in this society called the sabbatical. This ritual takes the form of a festival, in which witches get together and discuss recent events, spells, and other business. It concludes with a ritual that summons a number of demons from hell to take part of the festivities and fornication with the witches. The result of this night of passion is a cambion.

A cambion is a demon/human hybrid with magical abilities. They can be male or female and make up the population of witches and warlocks in this society. Because of their demon blood, they are unable to reproduce with normal humans. However, they are also unable to reproduce with each other, effectively making then sterile. This forces them to have to repeat this ritual every generation to increase their numbers. Why would this be the case?

Answer 1

They have spirits, not souls.

Let's go for a ride down a rabbit hole of European Christian fanon you probably didn't know existed.

Demons aren't biological

This one's just good theology from any perspective. Hell isn't a physical place that sits adjacent to the Earth's mantle. Hell is a state of being, and its native denizens are demons. Demons are angels by creation. Angels and demons are both eternal creatures, like God. They exist outside of time and have free will of their own. As eternal creatures, they saw and understood the Fall and Salvation from the dawn of time. In that timeless moment, they had a choice: "Serviam" or "Non serviam." I will serve, or I will not serve. The "serve" here is literally God, but the proposed hanging point is actually serving man. Because the angels won't participate in Salvation, but God will become incarnate as a man, the angels will ultimately be serving humans. Compared to angels, humans are weak in both will and body, limited in perception and wisdom, and frankly not cut out to tie Michael's sandals. And they're being asked to serve those things. Gross.

According to popular interpretations of Revelation 12:4, about a third of the angels were convinced to say no by Satan, and they wage war against the angels. This "war" isn't with blood and swords. That part's just analogy. Instead, they're fighting to bring about what the demons see as the proper natural order. They're fighting over the plan of Salvation, timelessly, as equal beings of equal free will.

Spirits don't create

Creation in the eternal time posture is an ability that only belongs to God. Spirits, angelic or demonic, have no creative abilities of their own. They can rearrange bits of matter like a smith crafts a sword, but they can't make new things.

Humans aren't spiritual beings, though. Humans have souls. Souls aren't eternal, contrary to some sloppy pseudotheological expressions to the contrary. They are created in time, exist in time, express their will in time, through some thoroughly handwaved interplay between God and man. Souls can give rise to new souls. We call this birth. It's entirely natural for souls to do this. Despite being as immaterial as spirits, they're really not related material at all.

Demons have an agenda

So, back to that war. The most common interpretation of the war is that the demons are fighting to stymie Salvation in general, to prove that they knew better than God all along. In my mind, an equally plausible explanation (and much better one for your story) is that the demons just want proper order. They're angels, after all. They like things to be a certain way, ordered according to cosmic spiritual principles and hierarchies. Mainly, they want humans to assume their proper place as subservient to angels and demons. The Salvation of man is pretty much irrelevant to them. Now, serving a demon instead of God means you're in a state of mortal sin and you're rejecting Salvation and you're going to Hell, but you made friends there, so this might not be the worst thing ever. You're a slave, but better a loyal slave than a rebellious slave when you're spending the latter half of eternity with control freaks.

They aren't particularly out to harm their loyal servants. Party with them once a generation? Sure. Pleasure is a good thing, and humans are meant to orient themselves towards the good. Grant them power? You betcha, because humans who orient themselves towards their proper end should have a place in the mortal hierarchy above the ones who don't. Have babies with them? Uhhhh...sure? It's going to get handwavy here. Maybe these cambions/nephilim are seen as an appropriate intermediary between the demons and the humans, because...

Cambions are their demonic parent, spiritually speaking

They're not animated by a soul, because it takes two souls to do that. They're not drawing their free will from a soul. They get it from a spirit made permanently manifest by hijacking the biology behind human birth. There's no reason to believe this would harm the demon any. Spirits are eternal and indivisible. You don't really get "more" or "less" of a spirit by having it incarnate. This is pretty much a great deal for a demon. Spirits don't fail to do what they intend, limits of physical incarnation aside. They don't have doubt. They're going to be oriented towards the goals of the parent, at least to the limits of their perception of them (and you could play this to any degree - there's no reason for them to be able to access demonic perspective, or not to be able to, or pretty much anywhere in between). They're also natural lieutenants of the demons in the temporal order. The demons would totally do this if the humans agreed to place the cambions/nephilim above themselves in their hierarchy.

You can soften it and say they're a mingling of the soul of the human parent and the spirit of the demon, but I say fuck it, go all in.

Answer 2

Note: as it currently stands, this answer does not meet all the OP's requirements.

(The OP's question requires that a cambion be able to produce offspring when mating with a demon. I had missed this requirement when I composed this answer, but it was brought to my attention in comments. I'm waiting to hear back from the OP on a certain point, after which I hope to amend this answer.)

They have a mismatched number of chromosomes from their father/mother.

This is already the situation with mules: https://en.wikipedia.org/wiki/Mule#Fertility

Mules and hinnies (female mules) have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.

Humans have 23 pairs of chromosomes, so your cambion inherits 23 chromosomes from its mother. Give the demon a very different number of chromosome pairs...say 66. Now your cambion has 89 chromosomes; many of which have no proper pairing.

I don't know if we have any mathematical models for predicting the chances of two cambions successfully reproducing given this situation, but if 1 chromosome difference is enough for mule offspring to be so rare, it seems like only a tiny bit of hand-waving to call it impossible for a cambion with a much larger difference between their paternal and maternal chromosome sets. (Most of the hand-waving would probably be that a cambion would be viable in the first place.)

Answer 3

Humans are blessed by some sort of god, granting them the ability to generate new life.

Demons have the dark ability of creating new life, too, mocking the god's ability.

Witches and warlords don't have any creative ability, and since they have demon blood they cannot rely on god's help.

Therefore the only option is to mate with a demon and let the demon half do the creative part.

Answer 4

The half-blood gametes of a Cambion contain a substance that kills both human and Cambion gametes when they combine. Pureblood demons have a recessive gene that allows their gametes to neutralize this toxin.

There are rare cases of pureblood demons with mutations in that recessive gene that prevent them from producing offspring with Cambions.

Answer 5

Male Cambions are sterile. Cambion eggs are incompatible with humans, but compatible with demons.

IRL, egg and sperm have a "lock and key" system to prevent cross species fertilization (to an extent, at least).

Answer 6

Demons are an addiction

Many people who live with an addiction feel as though they cannot function without it. In some cases, this can be a brutal experience: heroin, for example, strikes directly into the pleasure centers of the brain. In some cases, it can be physically true: theoretically the withdrawal symptoms from extreme alcohol addictions can physically kill you (although medical assistance can help you get through the worst of it).

What if demons are the same way? What if, once you've experienced the night of sabbatical, your reproductive organs literally stop functioning meaningfully except with the demons. Your reproductive system becomes addicted to something about them, and are infertile in all other circumstances. Such infertility is well within the realm of biology, and you can fall back on magic if needed to complete the story.

Such an addiction could arguably be inherited from mother to child. If your reproductive organs are addicted, and you have a little parasite growing in you which could get in the way of future fixes, your reproductive organs might lash out, crippling the child with the same addictive tendencies before they are even born.

As an author, I think you get to choose whether this addiction is something which can be kicked or not. It may be that nobody has had the strength, leaving the potential for a hero of the story to break free, or it may be that the addiction is physically more powerful than the victim, and there are no heroes.

Answer 7

Magic? You are talking about a creature that exists as a half-daemon as the result of a human reproducing with a daemon. Maybe these Cambions can't harness the same level of energy required to produce a daemon seed. Or perhaps the Cambions are not born of Human Mothers. Maybe the only way a Cambion can be born is by Daemon mothers.

I suppose this does not answer the specific question of why can't Cambions reproduce at all? I would be interested in the premise where they can reproduce but their offspring would be human and devoid of the daemon magics. Though this could alter the entire premise of the setting. Maybe the satanic society intentionally sterilizes all of the Cambion babies and it is a huge secret to prevent the Cambions from realizing offspring (to maintain control, or something?).

It could be as simple as the half daemon part of them mutates their biology to not resemble a humans. Maybe their reproductive organs don't work or they don't have them 'per se' in the way normal humans have them.

Just spit-balling here. If your world is more magical I say lean into the magic. If your world has a strong science aspect where daemon magic can be studied and understood by science then I would say lean into that.