Yuusha-chan no Bouken wa Owatteshimatta! - Season 1

After restoring peace by defeating the demon king, the heroine grows overconfident. She continues to dispatch the remaining demon army forces with ease, though she can no longer gain experience levels. Troubling dreams begin to plague her, in which she is overpowered and assaulted by demons. The sensations of pleasure within these dreams feel intensely real, gradually eroding her mental state. She eventually uncovers the origin of these nightmares, but finds herself unable to mount any effective resistance. The nightly visions proceed to corrupt her more deeply with each recurrence.