Ryuu Kusari no Ori -Kokoro no Uchi no “Kokoro”- - Chapter 1.8

The handsome, noble, cold, and ruthless family heir Ignatius Leclair took a liking to the fostered girl Delphine Carter in his home.From then on, the aloof man transformed into a loyal companion, taking care of children, cooking... The entire aristocratic circle was utterly shocked, with everyone envying.Delphine Carter smiled serenely: cunning, energetic, vengeful, a different persona by day and night.[Reading Tips: 1V1, both protagonists are pure, sweet story, the heroine is beautiful.]

Suddenly a high school student Subaru Natsuki has been summoned to another world on the way back from the convenience store. With the biggest crisis of his life being summoned to another world and no sign of the one who summoned him things become worse when he is attacked.

The Immortal Path led to Longevity and Sovereign was heading in ease. When the Longevity was no longer in ease, all living beings were just prisoners. Conspiracy, internal strife, and the invasion of the others, all of it emerged together. 

In such a world, there was such a young man. He made his way upstream, stepping on blood, defying the emperor, destroying the various kings, and slaughtering his way through the hundred clans to form an Immortal Path towards the throne of Sovereign! The Immortal Path was endless, while the Sovereign was limited. He used his blood to forge his immortal legend.

- Description from Novelupdates

"When will people understand.... I'm only half insane?"