Spoilers............
These will be a long rant so............
The whole thing with utsugi and sugi starts at ch 18 and its a complete 180 with en and ichii in chapter 17, and it goes on a darker turn,
Sugi, after the death of his husband, found a mysterious item in his ashes and has kept it in a box ever since, but the truth behind this item still bothered him for a long time. Sugi then found the right oppurtunity to confront Utsugi and Aogiri about the true nature of this item since they are known as the founders of the organization and his growing suspicion towards them. Utsugi at first is evasive of the subject and off topic starts talking about norse mythology, on how men are born from ash trees and it being a sacred wood that repels snake poison, that even snakes feared its shadow. Sugi only responds sarcasticaly to this story, commening on how fantasy-like it is, utsugi then replied that the “change” occurring within his own body is enough to make even myths like that
feel real.
Aogiri then interjected, and him and sugi then proceeded to have a back and forth conversation about when sugi was granted Japanese citizenship, They deliberately chose to assign him a tree name (japanese cedar) for it to be like a protective charm, but sugi continued to say sarcastic remarks on the topic. Sugi, growing impatient directly asks to get to the main point. Utsugi then tells him that the item that was in sugi s hand (the one he found in his husbands ashes) is the descendant of the blood stone.
The blood stone or the stone of immortality (ref. ch14) "breeds" through parasitization, every vampire is parasitized by the stone. It parasitizes the human body through the bloodstream, then altering the hosts body from the inside out with no regard for any side effects, it reshapes the body into something more “convenient", the result of that is immortality. The stone then drives the hosts to an overwhelming urge to consume blood for the purpose of spreading the blood borne infection to more people.
Utsugi stated that they struggle against the stone but even choosing to die is no longer easy for them. Since it traps the host’s very soul and sealing it away as one of the stone’s "children", a threat to itself. All for a single purpose and that is to "return", the stone wants to return somewhere. They are immortal vessels meant to carry the stone to its destination.
Sugi upon learning this is creeped out, utsugi then continued to say that megi (back in the day) and the other alchemists where decieved by the stone upon learning it isnt artificial but is living and has a will of its own. Its a truth that is hard to comphrehend, that’s the stone’s true nature. Sugi unable to process it all only replied "it feels like I’m listening to a movie script" and asked the question, "who are you? how do you know the truth of the stone?".
Utsugi then replied that he knows this because he has talked to the stone and is the only child of the Blood stone who has is permitted to converse with it. His true identity is hal luminox loratio, the immortal prince of a country that vanished long ago from the maps and the very first vessel of the blood stone.
Heres some of the lore about the mentioned country:
A cursed disease that clouds the blood and consumes the soul itself is said to have once silenced half of the continent.The curse even made it into the royal palace and those part of the royal family died one by one. On the night when lives were taken and the nation mourned.
A serpent handed to the king, the stone of blood and the stone then whispered to the king "i shall grant this country unending life". Everyone rejoiced in the blessing of the gods, whether by chance or fate, a life was bestowed and these where the birth of the kings' beautiful twins, but once again the stone whispered to the king "and give them to me".
And thats where ch 18 ended. There'll be more though on the next chapters.
Another long spoilers................
As for en and ichii, their chapter was very sweet and wholesome, but also a bit meloncholic. i'll just exclude the cute scenes since you just gotta see it yourself on how adorable they are, but i'll just share the parts of ch17 that seemed important.
En falls asleep on their trip to a vineyard (which i assume in france or somewhere in western europe or smth) where theyre visiting a winery to check on the new vampire character. on en's dream, the other people (which all looked awfully familiar) including himself is portrayed as a child version of themselves, where they are asked by strange figures to not be greedy and only pick out one gift in the christmas tree, only for en to pick not an item but a person. (Idk but this scene feels somewhat connected to them being the stones children and the strange figure is the stone itself talking to him). He is then woken up by ichii after them finally arriving in the winery.
The new vampire is named angel and has been running his family winery for hundreds of years, his backstory is that he ran away from home and his responsibilities as a heir of their vineyard, only to wander aimlessly in the city, where he drinks himself to death in the back alleys, one night he was attacked by someone only to wake up as a vampire. With him unable to come out in day time, drink alchohol or resist the urge to drink blood, he had think that he became something monstrous. after many years, he wanted to go back to the place he left and that was the vineyard, his capable younger brother saved him, shared his blood to him, accepted who he was and even gave me work. And after that he lived
on the farm together with his younger brother. After his brother passed away he became the owner of the winery, he then lived with his nephew and with his nephews children. Through generations, his presence has continued on the winery and still accepted as part of the family. Even through terrible times such as when people discovered his nature and burned down the vineyards in his hometown to kill him, his family survived with him and still continued to accept him regardless of him attracting danger due to being a vampire. now in the present he runs the winery with all the descendants of his brother, where they hide his true nature to other humans.
This chapter was very cute specially en and ichii s interaction but the tone of next chapter becomes very serious quickly and it seems the two of them will still be tied to the main story, particularly en but will take the back seat for a bit so that the lore gets more screen time.

Just finished reading ch18 and let me tell you most of the theories people are coming up with how fangs originated, none of them hit the mark, well a few of them were close but its no exactly what you thought it is, no wonder ichii and en in ch 17 was diabetes inducing , ch 18 was were utsugi hit you with the hard truth, i had the same reaction as sugi when utsugi was telling him their secret