Overlord (WN) - Volume 1 Chapter 67

Shu Yu transmigrated and became the original novel’s violent and brutal female Demon that caused the supporting male character, AKA hidden Boss, to become batsh*t crazy. However, right now, the Boss is just a lowly slave that was just given to her. What should she do?

- Description from Novelupdates

Amid the silence of the aftermath, a sealed envelope was found resting beside the fallen Infinity Blade. Its edges were worn, as though it had been carried for many years, yet the seal remained intact. Gok, known for his stoicism and strength, had written it long before the final battle, anticipating that one day his journey would reach its end.

After Xia Feng transmigrated, his heart was mysteriously removed.In its place, a defective demon heart capable of sustaining life only for eighteen years was implanted.This is a magical heart that can sync with the power level of its target.To stay alive, he bound himself to the ultimate villain of the future, a Destructive Witch who was destined to destroy the world.The stronger the Destructive Witch becomes, the stronger he becomes!Thus, Xia Feng realized, it's not my fault for stirring up trouble: “Miss Witch, I admit you're strong, but not strong enough!”