Legendary Youngest Son of the Marquis House

A brief description of the manga Legendary Youngest Son of the Marquis House:
I came here with only the goal of revenge.
I overthrew the Tulkan Empire, a single nation that dominated the continent.
I had always succeeded in gaining victory. I had accomplished all I needed to and was satisfied.
That’s why I accepted my approaching death. I no longer had a reason to live.
And so my heart stopped beating.
I definitely felt it stop.
However, what in the world happened?
“It’s not black magic and it doesn’t seem like an illusion either.”
A boy with short, black hair was reflected in the mirror.
I looked exactly like when I was 14, which was 20 years ago. A young boy, who hasn’t lost his baby fat yet, patted his cheeks and murmured, “How is this possible?”.
I had returned.
To the time filled with my regrets.