 If you liked
Shingeki no Kyojin
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 ...then you might like
Kamisama no Iutoori
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In both the charactares have to constantly struggle for survival. The origin of the threat is a mistery. SnK is more of a battle manga, KnI a survival game. Although labeled as shounen, they sometimes fit better in the seinen genre (like quite a few series in Bessatsu Shounen Magazine). Both are amazing reads and feel similar.