Just some things I want to point out about Blue Exorcist’s timeline and some theories. Spoilers ahead as it includes information from the most recent manga chapters.
So we know that Blue Night was apparently due to Satan’s “ego” sprouting as the incarnation of Gehanna.
(Which makes it somewhat confusing as to why the Demon Kings think of him as their Father when he’s younger than all of them. Technically. As a conscious being.)
However, Satan clearly had an awareness before the Blue Night. How do we know this?
Shiro retrieved the Koumaken, Kurikara, before the Blue Night.
Tatsuma and Shiro met while Mrs. Suguro was pregnant with Suguro (as in Ryuji). He was born in August so that gives us a bit of a timeline, but it’s confusing because it looks like it’s winter due there being snow all over.
Maybe it was snow in April or something?
Because another thing we know is that Yukio and Rin must have already been conceived.
How do we know this?
Because Shiro straight up tells Tatsuma that they’re going to use it to kill a kid. He got the Kurikara in preparation of destroying whatever spawn Satan produced.
So their birth was already anticipated.
(Plus, I’m assuming the pregnancy was meant to be the same length as a normal human pregnancy, though Yukio and Rin were probably born earlier than they were meant to. Because twin births are generally early.)
The Blue Night obviously happened afterward, as Tatsuma said that it was a few months later and that’s when he learned the truth about Karura. Though surprisingly it seems like it was still several months before the twins were born.
We know it was after Konekomaru and Shima were born (with Konekomaru in January and Shima in July), as Konekomaru lost the rest of his family and it’s mentioned that Shima’s eldest brother died protecting him that night. But we still have never been given a definite date as to when the Blue Night was other than it was 16 years ago.
(Again as the twins will be turning 16 on their birthday, this helps with the thought that the Night took place before their birth and that they had already been long conceived.)
So that makes me wonder all the more of what exactly went down during the Blue Night, because of its weird placement. It seems like it probably was Satan’s first incarnation “death” as he was obviously desperately looking for a new host by possessing the strongest exorcists. But what caused it?
Was it a natural degradation like what happens to the others’ host? Did Yuri have something to do with it? Maybe to protect her unborn child(ren)?
(I don’t think the anime version of their relationship will be canon due to Satan’s conception and the way he talks down about Yuri the first time we meet him. Then again, I could be wrong and he did love her. I’m highly skeptical though.)
In other words, in trying to figure out the mystery of the Blue Night and Satan and the twins’ birth, I am left with more questions than answers.