My Childhood Friend, Who I Thought Was Dead, Had Become the Demon King.

Younger childhood friend Demon King x Engaged female protagonist

As a nun watching over children left in their care, the protagonist Lucia lives peacefully together with her lover Orto.

Orto proposes marriage to her at last; but roughly seven years before, Lucia had a childhood friend named Nero, who was chosen to be the Hero and went off on a journey, and she could not forget about him. Seven years have passed since that day he left, and she decided to live facing the future giving Orto her answer, but...

In their town that had always been kept peaceful, a monster appeared. Lucia ended up following it by herself right up to the outskirts of town, and before she knew it she had lost consciousness.

When she regains consciousness in an unfamiliar room, right there with her is Nero, who's all grown up and who is now the Demon King.

This is a story that revolves around a fiancée getting stolen and the fully exposed, setting-things-straight sex that follows.

(Source: DLsite garumani)

Note: contains Netori.