Man, this story, it's very confusing. It seems to have ended now, but I hope there is a season 2. The end part, is probably the beginning now. They'll probably be adventurers, and then maybe the hero will get stronger and then she will gain an insane amount of immeasurable power.
The story is confusing, mainly because, well. There isn't much context behind Seika's power and how people view him as a demon king of the era. There's so much time skip and it gets confusing. Maybe the manga/LN/VN might have more context, but not sure.
The art is very nice in this anime, I do like some of the scenery, or maybe I'm based.
There's, well, very little character development, as there are time skips; an example being, one: A character dislikes the protagonist and the next, she likes him. And, we don't get to see much character development, just the usual. "I don't like you, protagonist, for these reasons" and then "I like you now, protagonist, now we can be friends" or maybe I'm based here.
Ah, I wish they let us see more of Seika's powers though, there are many other, onmyoji/spirits/beasts that he has not unleashed yet and I thought we see all of them that was shown in the opening, but there are some that have not been unleashed.
I think I may be repeating myself a bit too much. Also is marked as spoiler, so read with caution. I wrote this while I was half tired.