So... is Zetsuen no Tempest finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: March 29, 2013
Manga finished: October 12, 2013
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
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Synopsis
Zetsuen no Tempest follows Yoshino, who's secretly dating his best friend Mahiro's deceased sister, Aika. After Aika's mysterious death, Mahiro vanishes, seeking revenge. Yoshino is pulled into the conflict when Mahiro reappears, now allied with Hakaze, a witch from the Kusaribe clan.
Hakaze has been exiled by her own clan, who are vying for control of a powerful artifact. Her brother, Samon, threatens to unleash its power with potentially devastating consequences for the world.
Yoshino and Mahiro must now help Hakaze prevent Samon's plans from succeeding and protect the world from destruction. Meanwhile, they also seek to uncover the truth behind Aika's death, a mystery that seems to intertwine with the larger conflict.