Saikyou Yuusha no Boku ga TS Mahou de Maou Gachi Koi Mesu Inma ni Naru Wake ga Nai

"I'm the strongest hero, you know!
There's no way I'd lose, even against the demon king, right?"

Finally, at the end of their long journey, the hero Arwen and his party have reached the Demon King's lair. As they dive in for what they assume is the final battle, they see the Demon King and his succubus, Rutona. However, Arwen has underestimated Rutona's magic and finds himself captured and transformed into a succubus himself...!

His new body is just the Demon King's type, and he coerced to become a sex pet in Rutona's stead...!? Arwen's not yet used to his new form, but he'll do what he must to protect his comrades...

By the time Arwen realizes he's being affected by more than just his body changing, it's too late...

(Source: DLsite)