It hurts to move, so I overdid it yesterday. Better take it easy.
If the prophecy was that something terrible was going to happen when Izanami and Izanagi meet, Hei didn't save the world. A 3rd gate opened. Nevermind the boy-Yin, the prophecy came true in the disaster part. The OVA was an explanation of how they feel about each other. Yin has to be alive. Maybe it really was a happy ending. Hei chose to live quietly with Yin, achieving the paradise of no longer being used as killing tool like EPR wanted.
It would've been really better to show them walking hand in hand................
The sadness of the ending really stands out to me in that Suou-the-contractor died. Sure, the happy child version of her gets to live on the cloned planet, but the girl who felt confused and jealous over Hei calling her name and felt the pain of shooting those cranes... She's dead. Of course it's better that she live happily without having suffered the other memories, but she doesn't remember Shion, either.
What a painful show! I can't help but like something with a strong impact but the sadness of the 2nd season was more bitter and less sweet. Directly...maybe I should look at it as 'darker than black is always bittersweet, so they must have Yin and Hei living together at the end as the offset to all that bitterness of being apart'.
STILL CRYING -
Ooh there's another manga too, one more thing to look at. It takes place in the first season, though.