After Reincarnation, My Dragon Came and Smashed My Door

In his previous life, he was the most legendary Shadow Dragon Knight in the Azure Continent. He slaughtered millions of enemy troops with one man and one dragon, and eventually died on the battlefield. In this life, he was reincarnated as the disinherited third prince of a small border principality. The whole family was placed under house arrest by the regent sent by the empire. The other party claimed that he was suffering from a serious illness and would soon annex the principality until he "died" of the disease. His sister was also sent to the enemy's country for marriage, and his brother was sent to be a slave in the mines. Until his eighth birthday, the golden dragon mount in his previous life that was so arrogant that he refused to acknowledge him, and the dark elf attendant who looked cold but actually hid desserts for him every day, kicked down the door of his castle and said, "Master, we are here to pick you up."