Attack on titans is a Japanese series written by HAJIME ISAYAMA. It kicked off in 2009 and is still in production as of 2017.
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The story is about a boy EREN YEAGER and his friends MIKASA and ARMIN who's hometown is invaded by titans (gigantic humanoids that eat humans without reason) and Eren's mothers is eaten. Due to this tragidy Eren swears to avenge her and eventually joins the military to fight off the titans.
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As the story progresses, it unfolds a plot twist as Eren, who sworn to kill all titans discovers hidden locked off potentials within him.
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If you want to know more i suggest u view the summary video [AMV] link below
Its a very amazing anime, i 100% recommend you to watch it