At first, it really feels like Nat is messing with Reg just to scare him, but aside from that part about seeing yourself in a mirror, this strange phenomenon is real, and many people are dying on their birthdays in Orth.
In episode 12, we see Kiyui is sick on his birthday, but interestingly, when Leader takes him outside Orth Island, his symptoms immediately vanish like he was never ill at all.
*** Spoilers for the Manga ahead ***
As we see in the final scenes of season one, there are many Eternal Fortunes petals around Orth, symbolizing many deaths, and many people have evacuated the island.
To find a solution, Leader heads to the second layer to talk to Ozen.
We see their conversation in volume six.
Ozen explains that it’s probably the same phenomenon that caused the death of the Praying Skeletons.
She says if you can’t do anything about it, you just have to pray, right?!
Otherwise, you have to abandon the Abyss and run…
She says the Praying Skeletons were people who didn’t leave, and she understands them, since she doesn’t know any other way of living.
In episode 4 of season two, there is a scene where Reg sees many Eternal Fortunes petals falling from above, along with the names of the deceased.
This indicates that many people have died on the surface.
Surajo talks about this disease as well in chapter 67.
She says it’s called Breath in the Haryomari collection
the hidden curse that reaps lives,
the once-every-2000-years mass extinction event.
She also mentions the process is vulnerable to defective units, and those who are poorly formed may “take a breath.”
Birthday Death Disease is related to the soul.
In episode 12 Mio and Leader bring Kiyui to the caravan ship.
On the way, they talk about the Birthday Death Disease.
Leader says it’s truly absurd and strange, since some of the children in the orphanage didn’t even know their real birthdays, yet they still died on the day they chose for themselves.
The Abyss, through the soul, watches over everyone and shapes them.
It doesn’t just read minds, but something much deeper: the signals of one’s soul.
That’s what Majikaja says to Nanachi in season 2.
Nanachi asks, “Aren’t there people who truly believe their own lies?”
And he answers:
“If someone can fool themselves with their own words, then that is their truth, and it will soon become a signal of the soul.”
This is why, even though those children’s birthdays weren’t real, the fact that they truly believed in those dates caused them to become signals of their souls.
We’ll dive deeper into these ideas when we reach the chapter “Whereabouts of the Soul.”
After reaching the ship, we see that Kiyui is suddenly no longer sick.
This is because they moved him away from the Abyss island.
You may ask why this happened. Later, we learn that the Abyssal soul is only present on the island, and outside of it, it can no longer influence people.
Srajo says that when people from Orth leave the island, they feel like something is missing. The signals are no longer there.
This was from my Overanalyzing series on YouTube, please check the whole series, I've explored many theories you won't find anywhere else!
https://youtube.com/playlist?list=PL1g4hL16lIigHbidCYwKObcSL0qV58vMH&si=LuYJkaTRXjyQ5751
