Blue Exorcist is one of those anime that is greatly marred by anime-only arcs. Without giving anything away, suffice to say the last arc of the anime pretty much turns the manga on its head in a way that feels extremely hard to believe. The fact that we're getting another season that tries to pick up the manga plotline as if the anime's weird ending didn't happen only adds insult to injury.
A good anime for the first 2/3rds, which is based off the manga. I'm sad to have to mark it down so drastically for its botched final arc.