I think is appropriate to look at the bosses with a more indagative approach, this should probably raise some more questions, and hopefully some answers..

Lets first give a quick look at the whole, so we can go deeper later..

There are 6 boss in total, the first we meet is the Patriarch
We know that the boss in reality is kinda a vengfull wraith, or a damned souls, cause patriarch’s body, Frederic’s body is layng in his bedroom, mutated, but PHISICALLY there, so the boss itself, despite been touchable, cant be his human shell.

Wet han face the Lumen Caster, a huge humanoid figure that is rooted to a chair(or something like that). He wear a cloack and a hood cover his face, although time to time a single Eye can bee seen, after his barrier get temporarly disableb by the Corroded. We know (although i cant find the original source) that whoever he was before, he’s now possesed by the Lumen flys and controlled to serve as a “security system” in Necropolis

In the Sewer we’ll meet the Cloven that is actually an optional boss (if the player are not planning to do the true ending…but who want that? So i think is safe to say that Marrow only got ONE ending..and i dont know why i am even talking about this at this point).
We know he’s connected to the Cloven spwan, withc he probably give birth too by vomiting theyr larvae arand. Although im quite confident in my English, i dont really know what Cloven meant, and looking online the only answer i got was that Cloven refers to the animal endowed of hooves.
Now…the thing dont really have hooves,,but pointy legs and arms more like bugs, and to me the cloven really resemble more a catfish than anithing else ..so why is called like this i have no idea

The Imprisoned Seer is an interesting one, located in her hut at the end of malignance, is “kinda” a secret boss. A deformed humanoid with 3 eyes…sorry that USED to have 3 Eyes. Her legs are not visible, she drop marrows witch will spawn marrow clones of some lesser enemyes constantly. When defeated will give the Tendrill sword

Olnok’s Eye is probably the most trusted of olnok’s Haunters, he’s fuse to a wall and guard the last door protectinjg the Familiars’ prison. Is a face inside a snake mouth, two giant eyes over his head look deeply at the player, probably olnok’s himself, as the name suggest.

The Familiar member of a powerfull and immortal race, embodyes the Wanly will. The hearth’s Famialiar are an hybrid created using a Lumen Fly larva.

Ok with this little sum up, lets address what i think is a common almost all 6 bosses share. Lets breefly talk about the “Big Humanoid”. I’ll call this people “Proto-human” although i dont really think we are that close to them, but in absence of better name, since they do have an antropomophic shape… ill stick to proto-human. (@Vincent, you could.. i dont know…start to give some more clues to us :=) )
We can see this race, or what is left of it, mainly in the Red tower. The all have have the same look:
Pale skin, big head with no airs, pointy appendix. If we want to make a comparison, Daniel will look like the size of a Cat near them.

Why are they sooo important?? We kinda see them everywhere actually, one of them was used to give birth to the Familiari t self! How willingly that pregnancy was, we dont know, but we surely know that the birth itself was not pleasnt. The body is charred like fired consume it, arms and legs tied by chains (?) and a giant schyte was used to conclude the process. nontheless Vincend called this room : “Chamber of the Mad birth”.

A couple more are layng on stone altars, and one of them even raise up and eventually vanished, let us wander…

All this people, this.. “PROTO-HUMAN(?)” clearly lived at the time Olnok was still around and still experimenting, probably helping him.

The Imprisioned Seer look is very close to them, The lumen Caster size and ponty appendix can be a clue to indicate what is hidden beneath that hood. The Cloven have the same appendiz..and his size match!
Also some other enemyes and skeleton could have been a part of the proto-human.
Stuff like the Witness, the host etc,
But we cant forget the patriarch! Look at him, despite the humongus size he do look like a proto human, but he cant be one right? Lets start our deep analisys from hima gain

Like other bosses,he isnt protecting the area we found he in. I mean, the Stuland’s Manor is already well protected enough with monsters and spirits!
No no, good old Frederic is protecting something that he cares deeply, his Reservoir of Marrow!
I will post later another theory about the area of the game, so ill say it quickly here that i think the Reservoir is a more “recent” area, digged by the Stulands, not by Olnoks.

Frederic subtitle is (found on Vincet blog) “Abominable Conspirator” well DAMN! Thats something we can work with right?
We know he was a pursuer of knowledge, with little fear for the consequence of his action. His quotes can be describing something way less metaphorical than one may think a first glance, maybe he really DRINK marrow that spilled from the ground they dig mercilessly. At the end, when his “””human(lol)””” body is failing him, his mind, that can no longer forget and deny the truth, involontarly get to the next level, forget his past, become one with the eldrich horros and finally get the “illumination” he so desperatly wanted, thats why he got his 3rd Eye and become similar to the Proto-humans, another “seer” if you will.
Maybe the Protohumans were transfigured after reaching a certain level of “Clarity”.
Interesingly, in the second phase, when he lost his “clairvoyance” (his eye exploded with the top of his cranium) his attak become crazy, almost randomic, like a blind man that try to hit something in the dark.
Of course…it could just be thats a second phase, he’s mad and it need to become more dififcult.

Moving on to Subject number 2, the Lumen Caster.
If he is one of the Proto-humans, he’s the only one with the decency to wear pants! We can even see shoes. Again he’s not Protecting Necropolis, but rather the access to the laboratories, a way more important location, strictly connected to the Teleportation hub (again, ill talk about it in the next theory)But the thing that keep perplexing me is simple… why he’s hostile to us?
Now, think about it, the Lumen Caster is controleld by the Lumen Flies right? When he get in danger, the familiar presence even comes in his help, maybe since it’s half lumen flie, the Familiar sense his Half-kin danger. But so far the Lumen Flies were on OUR side! Even after the battle no much change! Sure.. we start killing the ones that soar around the harps, but thats it!
Lumen flies showed us the path, the secrets and the dangers of the old mountain, and keep doing that after Necropolis… so Why are they hostile there?
Another peculiar thing that the lumen Caster got, aside the clothing, is that he got TWO extra arms with fingers(claw) on his back.Were those wings? Vestigial wings? And lets not forget the Achievments we got by defeating him: “Castration”. Lets pretedn i didnt talk about this before, lets start anew:
he is a caster, so he is a magic user, probably more competent than others. He can be the ONE that perform the ritual of the “Mad birth” under Olnok’s supervision, finding or summoning the Lumen flies. In that regard he’s the surrogate father for the Familair, and since he’s power, despite been controlled by the flies, is gone with him, is no longer possible (not the someone will want to) to repeat the ritual and Create another Familiar, and so “castration”.
But why he’s hostile? Better, since he’s controlled, why are his lumen flies hostile to us?
One possible and quick answer could be that they are acting in authomatic, attacking everyone on sight. Simple, yet possible, we dont really need a convoluted explanation for everything..

…but lets say that we WANT to XD
And here is were the Stillborn comes to play.
Stillborn are strictly connected to the Lumen Caster boss fight, we need to “shoo” em away from the Corroded in order to break his shield ( i wojnder what will happen if we dont) and more Stillborn will try to attack him/her again tryng to stop him to do so-
And lets not forget that 2 more small shoting Stillborn can be summoned against us.

Vincent says this about them: “Stillborn is a mystery for the most part. They reveal themselves early on and permeate the entire game. Their skin is acidic but whether or not they actually intend to do harm to Dan is unknown, and unknowable. They are alien in that they don't originate on our perceptive plane, though that doesn't necessarily mean they are from outer space.”

We can also see em in the laboratory, suggesting that someone make em or at least experiment on it.

So here’s my theory: Stillborn are Mutated Lumen Flies. We dont know where the Lumen Flies comes from, and they happen to be a very important creratures! So powerfull (in what regard we dont know) that thye have been choosen for the Creation/evocation of the Familiar! Also i think they are very much connected to the Resonating orb (check my Theory X)

How this abomination happened we dont know exactly, maybe marrow exposition, or maybe been inside the Caster for so long corrupted em, but in the end they Degenerate in another “species” of lumen flies.

Case 3: The Cloven
After a deeper investigation (checking wikipedia XD ) Cloven refer to a “Specific” type of hooves, the one usually Goats posses. But.. i m really lost about this boss, i can really figure out how much “sentient” the Cloven is. What do we ACTUALLY know about this boss?
-He is BIG. Ok all bosses are
-He can live inside pure Marrow. Litterally a river
-he’s protecting the access to Malignace
-He have small (primitive?) magical ability
-Despite the many pointy legs, it’s appearence is the one of a Catfish rather than anything else. Big circular mouth, long mustache, small tendrils around it.

Interstlingly, Catfishes happen to be a very strong and resilient creatures, they can feed on allmost everything, can grow (theoretically) infinitly bigger and bigger.

Examining his Spwan we cant really get much more info. They have a similar face, but just 2 leg/arms that they use to crawl around, but since they can reach very high places, they probably have some sort of sticky mucus that help them to climb. Also they have also moved far away from the sewer, so or have a huge sense of exploration, or want to stay away from theyr Father-mother as much as possible. Last thing they can drool little pellet on Daniel in order to hurt him.

We got immediatly ambushed by this boss when we leave the docking area. Since a very big rsisting-marrow boat was setted there, we can safely assume that the Marrow river was there for a very long time, and peoples…or considering the size…the Proto-humans needed to traverse it quite often, did the cloven generate after everything was abbandoned? Or he was a loyal pet that they know how to deal with?

Personally.. considering that the area is called “the Sewer” and not something like “true reservoir”, i think that whoever was there, really toss theyr waste and garbage into the marrow river for some reason..
The Cloven was some sort of animal that was living of such waste and miraculously survived the Marrow exposion.

Why Cloven? Uhhhhh.. the only thing i can think of is: cloven are bifid pointy hooves.. this boss appendix are probably hooves-like, and since he got a lot all over someone named it like this.. sorry thats really all i can think about

Case 4: The imprisioned seer
This one, is probably the boss i can have more to say about
Ill’s start sayng that the Seer is a member of the proto-humans, She got too many trait in common to be ignored: Pale skin, huge inflated head etc
She possesed 3 eyes, and how her name suggest, she can see…she USED to see the future and secrets through extrasensorial sense (the third eye i guess…).
But there is more! Her title is “Unwitting Beacon”, and in her lore Olnok(?) hoped that all the thing he did were anough to “silence” her. But lest leave this aside for a second, there is one cool thing i ‘d liek to point out first:

We kill the Patriarch..anb he vanish, was probably a ghost right?
The lumen caster, fade away..,
The cloven get incorporated inthe river of Marrow,
Olnok’s Eye fade away like the lumen Caster..and the familiar as well..

The imprisoned Seer, is the ONLY boss witch corpse remain in place for us to examine (until we left the arena..) a deflated and lacerated corpse. Much like a baloon after get popped. Was she actually the only phisical boss in the entire game? The one that despite everything was STILL alive???

Back to the boss, we know from here “lore” that Olnok actually knew the Stulands and the Patriarch personally

“My hope that the terror of Frederic’s wake will be enough to ward off any curiosity, before any travel further below”

He could have say “the patriarch” or “ the Stuland family” or other sinonimus, but he call Frederic by name.
Also he probably have given him input to do his own experiment explicitally to do damage and discourage others to dig in his mountain..but that for another time

What strikes me was that Olnok wanted to “silence” the Seer, so he carve her eyes and hide them, mutilate her, probably cutting her legs, and imprisoned her in a secret place. But…why dont just…Kill her??

I come up with 2 possible explanation: one is more “romantic”, in witch despite all what he done to her in the vaine effort to stop her nature of been a Beacon(one moment and we’ll talk about that) he cannot made himself kill her. He felt guilt for all the pain he inflict in vaine, and cannot force himself to do more..

Buuuuut i think this other explanation will suit better:
He cannot kill the Seer. At the time, the “Technology” of the reflecting orb was not yet discovered, and the seer is immune to all source of damage but her own, plus,i state above that maybe the Seer was the only real living creature (as we intended at least) left in the old mountain.
But she…bleed Marrow! Was her infected? Or… Was she soooo connected to the fabbric of reality that actually hurting her would make the universe bleed, and therefore produce Marrow??
Other creature will eventually produce Marrow after getting infected, so it wont be that strange… but she didnt look infected, she look like the other Proto-humans (beside been mutilated).

And what about her title: Unwitting Beacon ? Beacon…for what?
was she the “light” that the inhabitant of Adit followed to reach us? I think so! We already know pathfinders have come from Adit thanks to Exoder, but more creature that we can spot in the game are very close in appearence to the one in marrow like the Spitter! So probably Olnok uses the Seer’s power for his pourpose (contacting Adit, find a source of marrow or other stuff ) and notice the constant coming of Alien unwanted monsters andwhen he realize he cannot stop what he started he imprisoned her.
Although a few crawler and spitter dosent look that bad, maybe he had to deal with some more dangerous struff.. like the “blue guy” that Stop the teleport in the final chase (we also got a picture of it in the stuland mansion upsidedown room) Final note..olnok talked about beaconS.. so more than one

There are a couple of more things that intrigue me about this boss that i dont have a clear mind about it:
-The Tentrill Sword
-The instant exit of her arena
-The City model with the laser Indiana Jones style.

The building depicted in the Model have a strange geometry, alien if you would… and we can see some of those, probably in ruin on the landscape of Cinder. Was Cynder the home of the proto-humans before?
There i salso a “light house”i think windhime or honoured refuse..i dont remember.. is the “star” that we see in the dark that show us the Luminous orb code to use in order to summon a bridge and let us safe an orphan. Was that the Seer’s hut? I say light house, buti t could have been just a tower (cant be the Red tower, since is very close to the surface).

We traverse a small area before entering the Seer’s arena, nothing special really… but instead of doing it in reverse after killing the boss, we find ourself back at the Hut. Was that a dream or an illusion? Not sure

The Tendrill sword have an “alien “ look to it, and it is dropped by the boss after her death. Is actuall ythe only weapon that dosent lay on the ground, abbandoned by someone else before us. Was it inside the Seer?
The description made very clear that this weapon is not from our world. “hard to grip and gleams with an otherworldy glow”. Were the Seer and the Proto humans all alien? I cant exlude this possibility, probably the city show how theyr world looks like.. but how they reach hearth? And what she was a beacon for than?
Why the tendril sword appear there.. keep bother me though.. maybe was just a design choise, with no lore attached..

Who am i kidding?! i will keep obssesvly thinking about this ☹

Case 4: Olnok’s Eye.
Lukly for you, im almost out of stuff to talk. I was told Olnok’s eye was one of the haunters…but can find any official source for this. The only thing i can speculate on is if he was autonomus, or controlled in real time by olnok himself, not really worth of a discussion.

Case 5 the Familiar:
There isnt much to talk about this Creature, we dont need to speculate or search for stuff, since we allmost know EVERYTHING about the final boss thanks to Vincent and clear clues in game.