It has a long name. It is about the demon queen basically deciding to marry the first hero that breaks into her castle and search for a stop to the never ending cycle of their demon/human war. There are many renditions of the manga form, but sadly, they chose the least processed one that clearly focuses on a...different kind of "plot" if you catch my drift.

But what's it called so I can watch it

Maoyuu Maou Yuusha. That is the basic manga name. The version I like has a much longer name.