I will make it short. This is definitely one of best arc of this series, and you definitely should not skip this arc because this arc tells us about the past, the origin of Satan and Rin & Yukio 's family I've been waiting for so long and finally can see this arc being animated, and I'm glad it's finally here. They didn't include some scene from the present, but I think they did it because they don't want to ruin the story so they focused on history from the past. Maybe they'll bring it up in next season. Overall, they delivered the story so well that they canbring the feeling that I feel when read the manga.