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Q: What is a Nephilim (in Darksiders lore)?
A: Nephilim are the result of combining angel and demon dust.
It is not stated as to whether or not this is actual dust combined via magical means or some sort of euphemism for sexual reproductive fluids mixed together in an incubation chamber like little test tube babies, but we're dealing with Lilith, and she is a woman who is involved in STEM per her massive, hidden laboratory. This could go a multitude of ways ranging from asexual to sexual reproduction. Let's start with the asexual method.
Asexual reproduction:
If we look at different examples in history such as Frankenstein's monster and religious use of magics to make guardians, I would say the nephilim count as golems (specifically dust golems, if you want to take the whole dust thing literally).
That means the way to make more of them would be through alchemical combinations of those elements, and thus that would be a perfect way for the nephilim to asexually reproduce. They just literally build more of themselves. If you don't take the whole dust thing literally, one could argue that there was a Frankenstein-esk assemblage of angel and demon parts (sort of how the abominations came to be in Abomination Vault), and that would still keep it asexual in nature.
Let's move to the sexual reproduction.
Sexual reproduction:
Nephilim could be assumed to have the same parts as humans.
It is mentioned in the comics that Death probably has male sex organs when Abadon threatens to "cut off his manhood". This also would most likely extend to his siblings for their respective binary genitalia. Lilith also mentions in Ds2 that "from the first, were not the rest formed" (alluding to possible sexual activity) when she encourages Death to bring back the horde, further strengthening the argument that the main way of getting more nephilim was through sexual means. They also have bellybuttons, which implies an umbilical cord connection (though if that is to another being or some feeding apparatus organic in nature is up for argument).
There is also some secret third option.
Combination reproduction:
I feel like the best option is a combination of the two methods. Should there be a lack of materials, sexual reproduction could be an option, whereas if there were a surplus of materials, asexual reproduction could be an option. It really depends on the needs of the horde at the time and what they have available to work with. We will come back to address this hybrid option more when I get to the top of different kinds of offspring.
Personally, it would make sense for the First-born to be the only true golems if we kept keep in mind that line from Lilith. Maybe Lilith was involved with one or two of them just to keep them under her thumb. She is attracted to power and the two most powerful nephilim that come to mind are Absalom and Death. We do see that she and Death have some sort of history in Abomination Vault that he seems greatly opposed to addressing, but those hcs about their relationship are for a different time (they're a little heavy and painful).
Q: Can they currently reproduce?
A: Most likely not.
E: The nephilim were a prolific race of brutal war hungry beings. They were called "the Nephilim Horde" for a good reason. In allowing them to reproduce, there is a great danger in that horde coming back again. That is where the Council's involvement comes in.
The Charred Council is very well known for having a knack for dominating and controlling other races, even going so far as to make the Watcher race, a slave race of shadow/smoke sprites that enact their every order and that they kill on the slightest whim, be it to make an example to others or as punishment for failing to fulfil and objective (shown in The Abomination Vault and the comics). They also are not above using agonizing magical methods to control the horsemen such as when Death disobeyed them and he was put into massive amounts of agonizing pain, even greater pain than thrusting his hand into still flowing lava (Abomination Vault).
Personally I wouldn't put it past them to make the horsemen sterile one way or the other. It could be physical mutilation and/or magical means. They were also the ones to directly remove the knowledge from Lilith for how to make the nephilim in the first place. They do not want the horsemen, or anyone for that matter, to be able to make more of the nephilim or for them to have power that they cannot directly take away by any means. The Council are very controlling and want no opposition.
We'll get into all the ways they could ruin children for the horsemen in the next section should they have let the horsemen make an attempt at having offspring.
Different types of offspring:
To me, depending on the sort of route one could take to get children, there would be a few different options on what the offspring would be considered. I've also taken the liberty to change the definition of these words a bit to better fit the themes in the story. If they can do that with the word Nephilim then I can do that with these words lol
Homunculi:
The original definition is something like "little man" (in latin) and means that of a small but fully formed human/humanoid being. I think I also read somewhere that homunculi could also be a small lump of flesh? (I may be misremembering, but I'm going to use that misremembering in the bad outcome in a bit, so let's run with it.)
Given that the nephilim were essentially nomadic (not by choice), being pregnant would be a huge inconvenience at times. Maybe nephilim were engineered to have a very quick gestation period and their children grew very, very quickly. I'm talking maybe a third of the time it would take regular human children to mature to adulthood, hence the whole homunculus definition. They don't come out the womb thinking and swinging, but they hit those marks in record time compared to that of other races.
Maybe we wanted to go the bad route with the idea that the Council didn't make them infertile per se, but made having children the worst idea in the world. Maybe they made it so should the horsemen accidentally knock someone up (or get knocked up in Fury's case) the baby would come out as some amorphous lump of flesh that is constantly in pain and only screams and howls. It not only deters the horsemen from having children, but from also having relationships with others in that way. It could also be a way of humiliating the horsemen like "see, look. Even you're offspring are horrific things that can't stand being alive" or something terrible to that degree. It would be a mercy to kill it so that it wouldn't have to suffer the pain of trying to live.
If it didn't come out as a lump of flesh, then the offspring wouldn't be exactly normal. It could be a situation where there's a bad blend of the nephilim and the other parent resulting in a mutant-like appearance. Think Jinenji from Inuyasha. He's not a monster exactly, but he doesn't look like either parent and is an outcast for being different.
Chimera:
This is from Greek mythology and is used to mean "a fire-breathing female monster with a lion's head, a goat's body, and a serpent's tail" or something that is wished for but is impossible. This also refers to a being made up of a mixture of tissues through things like grafting, mutation, or fusion. (I suppose that the grafting thing might also encompass Frankenstein's monster, but I still feel like he's solidly in the golem category. It's probably splitting hairs at this point).
This word could refer to creatures that are not other nephilim that the nephilim have tried to reproduce with, sort of like how "demi" can be used when a human is with something not human ("demigod" is an example). The angel and demon parts may result in split features of the offspring and make more hybrid babies.
The worst way to take this is having the offspring be literally animalistic in nature like how in Greek mythology sometimes a god would have a baby and it's an animal-like monster. These would be undesirable because they're actually monsters with animal like intelligence. Maybe they're magical monsters that can speak, or maybe not. Depends on how far one would like to go with this idea.
Either way, I feel like this would be the word to describe literal monster offspring with animal or above intelligence.
Golem:
In Jewish legend it is a protector figure, often made of clay, brought to life via magic. I feel like the nephilim (at least the first generation) are actually golems and so this could be a viable option for those who know how to do it to make more.
We know the nephilim had access to magic, and Lilith was there for some of it. She probably taught them how to multiply in various ways, and to use magic to bind materials into more nephilim. It could also fall into the combination reproduction method and they "evolved" them or imbuned life into them (think Bleach, where Aizen makes arrancar, meets Makers putting their souls into stone).
If the council wanted to deter this as the prime way of getting more nephilim, they would strip that knowledge from all who know.
This is the exact sort of magic that I think would be known to only a select few in the same way as how to make the abominations. If we take the previously mentioned line from Lilith about more nephilim coming from the First-born, then we can safely assume that Death knows (or rather did know) how to make more of his own kind as well and was one of the ones that had that knowledge taken away.
Now Death has an incredibly strong mind as seen in Abomination Vault where he successfully (when he was aware of it) blocked out the abominations mind probing. He could've thwarted the Council to some degree in the same way he withstood their punishment. Although, I don't think he got away entirely unscathed. He make have some inklings for how more nephilim could come to be and just not tell anyone, but he also may have a lot of pain surrounding that memory.
Maybe the Council made remembering so painful that it causes physical agony, or just erased that memory entirely and replaced it with a blank haze that makes him hurt all over. This could also apply to the others because they had to know something about how their own kind is made. All of them have a hard time thinking about children, not because they don't want any, but because they hurt when they think about even the idea of it.
Q: Do the horsemen know they're sterile?
A: Most likely.
They might not or they may have found out the hard way through malformed offspring mentioned in the previous section. I think that would truly traumatize them if they were unawares. At the same time, they knew what they were signing up for at the time of doing it. Maybe that was mentioned somewhere in the exchange for power.
War and Death I think would have agreed. Strife may hold some pain over it, but what's done is done. It is unsure if Fury truly cared to have children of her own. At the time, she may have been so absorbed with the thought of gaining power that it hadn't hit her until later. If that's the case with her, she may have been the one to find out the hard way before the others. Strife might have too, and it could contribute to his deep distrust and ire for the Council.
War and Death would never attempt it. War wouldn't test it because he made a deal, and his honor keeps him from it. Death would know that something would happen, and he wouldn't wish that upon his own flesh and blood. He also probably was the one to find out about Fury's attempt, or maybe Strife's. Death knows it's a bad idea.
Q: What if there was a good route that worked?
Now suppose it goes right and they can reproduce. The only species that might be viable due to plot and cosmic destiny/foils would be humans. Very similar size, shape, and even themes: the creatures that are made for this world paired with creatures that were never meant to be. Their children could be impossibly strong with humans being a blank slate for them to amplify their strength through. Humans, known for their intellect, mixing with nephilim, known for their strength. Something made by god in love with creatures made by one who would mock god. It's too perfect y'all.