Quote Originally Posted by nonsensei View Post
Not much is explained about them, certainly, but there's a part in the last realm of the main story where we get a bit more background about them. If my memory serves me right, it was when they met up with Satanachia. She left Diabolos' realm because they had to use seal from the inside & the outside of the realm as well to completely seal these disasters away. But that's about it, I don't think there's anymore information mentioned about them.
That's because DMM just stopped making the main story. It just ends after

Satanachia dies

.