I started watching this anime back in 2013. The soundtrack and visuals were breathtaking, but what got me hooked was its shocking cruelty and fatalism, something hard to find in the shounen (young male) demographic.

The story showed a world full of mysteries, in which friendship and willpower would not be enough for the characters to overcome their difficulties. The fantastic elements allowed epic battle scenes, but there were restrictions to the power scale that gave the feeling about a more thoughtful world-building in which unreasonably powerful characters, quite common in shounen, were not possible.

As the story goes on, many of the mysteries are explained, keeping the viewers hooked, waiting for more to be revealed.

As any other story, it had its ups and downs. However, it started losing its charm after the main breaking point. To make things worse, the studio that adapted the last story arcs didn't do a great job. The animation quality wasn't good, not even for the studio's standards. It lacked details, like textures and a good lighting. The production was rushed.

Nonetheless, that's not the main issue, but the deficient storytelling of the final arcs. The main characters lacked development, contrasting with greatly developed secondary characters. The drastic personality changes experienced by the protagonist were molded according to the events in order to justify them, instead of the other way around. That is, the changes were forced. The author missed his chance to deepen into the psychology of the protagonist.

The story also started falling into the most common shounen clichés, going far enough to break all the rules and logic set during the beginning: plot armours, deus ex machina (more precisely, enemy ex machina), illogically powerful enemies, loose ends and disappointing lore explanations with the addition of an attempt to introduce some symbolism to it. In other words, the author chose to take the easy way out of the labyrinth he built. The final events aren't the issue, but the way they were reached. It was like watching a racing car reach the finishing line completely out of control.

No matter how hard it tried to avoid being the stereotypical shounen happy end, it still failed.