The characters and intelligence is some of the best I've ever seen throughout all 3 seasons, and the music and art of this season specifically is also some of the greatest and a huge improvement from the last 2 seasons.
I don't want to say much about the story so I don't spoil anything (Remember when you spoiled Undertale for yourself), but I guarantee you'll be surprised, no matter how good your predicting ability is.
The anime sneaks in some atheist hot takes. The main villain is called the God of creation, and the main hero (Shana) bares a striking resemblance to Lucifer. The show will point out the fallacies of this God's and other ideologies and gives you it's own, smarter takes on life. You'll enjoy Shakugan a lot more if you look for them
I've heard alot of people say Shakugan's ending is their All-Time favorite but to me it just flies bye, better than most anime but disappointing for what you expect from Shakugan.
The major problem with this anime is that the type who would like it aren't the type to like the first 2 season, and vice versa. However,if one thing is for certain, this anime doesn't get nearly the attention it deserves.
