 If you liked
Maoyuu Maou Yuusha
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 ...then you might like
Maou Gakuin no Futekigousha: Shijou Saikyou no Maou no Shiso, Tensei shite Shison-tachi no Gakkou e Kayou
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Both have wars involving demon and humans
Both have a demon lord and a hero seeking for peace and becoming allies/friends
Both demon lord and hero live concealing their real identities (in a way since in Maou Gakuin no Futekigousha, the demon lord Anos was not really acknowledged as a demon lord.