Mrthfx

Angels were all originally part of the same race before the war in heaven, when the ones that rebelled were cast out and sent to hell. The humans on earth were similar to angels, but without the essence that made them divine. Among the angels that remained in heaven, some took a fondness for mortals and bred with them to produce mortal offspring called the nephilim. These creatures were monstrous giants that were bestial in nature and appearance, committing evil acts like cannibalism and murder. They were eventually wiped out, but the lesson learnt was that divine and mortal essences cannot mix.

However, this appears to not be the case. Devils, angels that were cast down from heaven, are still able to breed with mortals, producing cambions. The resulting offspring is still a monster, committing evil for its own sake. However, their bestial nature is internal rather than external, as they look completely human. Legends speak of the anti-christ, a being who would manipulate world governments and turn mankind to evil acts that have yet to be seen. This being would be the son of Lucifer, once the greatest of angels and responsible for the war in heaven. He now ruled hell, as he was cast into it with the angels that rebelled with him.

How is it that devils spawn monsters that look human, while the good angels produce monsters that look like monsters?

Here are two creature of the same species:

If you and most of your kin looked like the first picture, you would probably consider the second picture--10 times larger, with an un-naturally slim face, threatening ears, loose jowls, and weirdly short and dark hair--to be a monster.

Very similar underlying genetics can cover for quite significant variance in phenotype.

Hell is a pretty toxic environment. Being there over a long period of time has mutated the demons, causing them to look different from angels and passing on changes to their offspring.

First a biblical answer and then a fictional suggestion. Use which one you like!

I think the consensus of biblical scholars is that the passage about nephilim is super weird and hard to be sure about. But it seems pretty clear that it's referring to fallen angels doing stuff. As for the antichrist, in other verses (Matt 24:24, stuff in 1st and 2nd John, etc) basically any spirit that denies Christ is the anti-christ. There are theories that the antichrist mentioned in Revelation was actually Nero, who seriously persecuted Christians. See for Christians, we've been living in the end times since Jesus rose.

But yeah! For a fictional suggestion... It's a really big thing that Satan is a liar. It's so much a part of what he is that Jesus said it's his native language. So the demons' monsters are monsters that are lying about being monsters. The other ones are honest about it, at least. But it's gonna be hard to explain why an angel following God would want to make a monster lol. Hope that helps!

Angels still bear the blessing of good, while human and demons not.

When angels and humans breed, the interaction of a non blessed with a blessed cell messes up the appearance of the result being, while the pairs blessed-blessed or non blessed-non blessed give no issues under this point of view.

First we are to assume that Angels and Devils can be classifiable by species.

Second that they are the same taxonomic species.

Third that humans are of a different species?

Clearly then the answer is that it is something external to their taxonomic definition that allows interbreeding, as (going by our definition of species) neither could successfully breed with humans.

If it is an internal, integral and automatic capacity, neither Angels nor Devils can be classified as part of a species at all because they can breed with things other than themselves, so to speak.

In summary: However it occurs, to define Angels and Devils by specie must necessarily be flawed.

It follows anyway, that whatever agent it is that makes their breeding with humans possible is altered either as integral to their displacement(from Heaven or to Hell) or as a separate function imparted to their nature, rather than that of the agent. In real world terms it's theoretically feasible to create an egg or sperm that 'pulls apart' it's counterpart and reconstitutes it in a form viable for reproduction making ad hoc inter-species breeding possible.

If we consider that what makes Angels Angels must be 'impartable' only from the Divine, otherwise the Devil would simply make himself and his Devils undevilly, it would perhaps stand to reason that an Angel cannot simply make more Angels via mating, as it is not up to them to make more divinities, but the Divine. It might follow then that it is relatively easy to make more evil, but not so easy to make more good, so to speak. Heaven has an immigration policy to uphold after all, and free passes would make a mockery of that.

Let's start with the premise that devils are, at their essence, corrupt angels. The corruption damaged the very core of their being - in human terms, their DNA. Not all changes to DNA are destructive; most are benign. Skin color, height, and even facial features come down to DNA. In devils, the corruption they experienced caused their appearance to become that of a typical devil, mutilated and horrible.

Angel and Devil DNA appear to be compatible with human DNA, at least in the extent that humans breeding with either results in viable offspring.

The offspring of angels is powerful and terrifying to behold, just like angels. And, like angels... have no sense of morality. Angels, of course, are good at their very core; they naturally tend toward "goodness," and never think twice about doing the "right" thing. Humans, however, have free will, and the lack of a moral compass (coupled with a horrifying appearance) turns them into monsters, inside and out.

On the other hand, Devils, having suffered corruption to their very core, have very different offspring; the corruption they suffered damaged their physical looks so much that any genes contributing to looks are entirely recessive. The human genes, being dominant, take over, resulting in offspring that look completely human, or at least close enough to pass a basic visual inspection. Of course, they still suffer the same lack of moral compass, as half-devil is just half-corrupted-angel. But, as they look human, their upbringing would be much gentler than that of a half-angel, which means they would tend less to be entirely monstrous... and instead lean towards plotting psychopaths.

It depends if you're wanting to be non-religious fantasy or a religious based fantasy. (AKA, based off popular views of the religious entities in question, or the actual old religious stories about them.)

Not Religious Fantasy

You can have it simply be different breeds or subspecies. Many species vary wildly across individuals (think queen bee vs worker bee or Great Dane vs pocket poodle.)

Religious Fantasy

However, if you're going religiously based, angels and demons are not directly related. Biblically speaking, they're two very different creatures (possibly different species) and nowhere in the bible does it claim they're the same. It does, however, imply that fallen angels join demons to fight along side them (although, interestingly, no demons every rise to side with God).

Before the Bible was consolidated into one book, the origin of demons was made very clear. All demons are specifically the children of Adam and Lilith, as opposed to humans who are the children of Adam and Eve. You'll notice in the bible there are two creation stories: First is the 7 days story (where god creates man and woman at the same time), and the garden of eden story (where god just puts man in the garden of eden, and then makes women from his rib.) What happened to the first woman was left out when the various works that make the bible were consolidated into one book. The first woman, Lilith, didn't care being told what to do, and refused to have kids with Adam. This angered god, who then chained her up and... well.. god got what he wanted from Adam and Lilith anyway, without her consent. However, Lilith, being so treated, made sure her kids would know exactly what she thought of God.

The children of Lilith were the 'original' humans by design. They had both from-scratch male and female, and were fully human. Eve, on the other hand, was a copy of Adam, made from his rib, and made female. So Adam and Eve's children were only half of what humans were supposed to be. Further, the serpent who tempted Adam and Eve was supposed to be one of Lilith's children (the story about it being the Devil wasn't added in until King James).

This means that Lilith's children managed to get access to both the fruits of knowledge and fruits of life before god kicked everyone out of the garden. And Genesis 3:22 makes pretty clear that's not what he wants and why. So god basically declares war on Lilith and her children.

As they're the "full" humans, fueled with both divine fruits (unlike humans which are only "half" humans with half of the divine fruits), this is where demons got there name.

The "mon" suffix as in "Human" (remember, not originally english)
and the prefix "Dei" as in "Divine". How it was originally pronounced may have been closer to "Diman"

Demon literally means "Divine Humans", and are supposed to be the epitome of what humans could be, and were striving against God as a whole.

Obviously, when putting together the collection of works to make the bible, this makes demons look a better, and god look a lot worse (as forced intimacy and genocide are generally frowned on), so it got conveniently left out. However, if you want dig through all the old Jewish Midrash and more, it's referenced fairly often.

Although Hebrews at the time, when the story of Lilith got left out, still understood Demons and Angels were different species, this made things much later for later generations and Christians who picked up on the text. They tried to explain why there was such animosity between god and demons, and so it was crossed with verses about fallen angels joining demons to try and explain it away. And although it's not in the bible anywhere, that explanation has kept going through Christian tradition till modern times.

Here's a diagram if it helps:

Possible exception:

Now, that said, the Bible ALSO says that mankind is made in God's image. If we interpret that a different way than Christians traditionally do (basically meaning instead of a self-portrait, that humans are God's offspring), then demons go right back into being the same species as humans, and in turn, potentially angels and nephilim (and god himself) as well. Considering the ability for all of the above to have offspring with eachother, this actually seems a moderately likely interpretation: That God, Angels, Demons, and Humans are all different breeds of the same species. In this regard, we're back to the different breeds or subspecies being what creates different appearances again.