I Regressed As The Duke is a Manga/Manhwa/Manhua in (English/Raw) language, Action series, english chapters have been translated and you can read them here.
The Summary is
The son of the great Emperor Gline,
Prince “Aaron”, is the recipient of the “Dragon’s Blessing”.
But because of the rebellion led by his uncle, “Zerone”,
He was banished to the outskirts of the kingdom, ‘Brahn Grounds’.
“Emperor Zerone will kill us all…”
Keeping his father’s dying wish to forget about the crown and revenge,
Aaron had been living powerlessly as the Duke of a barren land… but one day,
Emperor Zerone invaded Brahn Grounds!
He didn’t come just for Aaron’s life,
but rather, his aim was to destroy Brahn Grounds!
Even the loyal subject, “Gayle”, disappeared into ashes
as he watched Duke Aaron and Brahn Grounds disappear
in midst of Emperor Zerone’s flames.
At least, that’s what we thought happened!
When he opened his eyes,
Gayle found himself in the position of a younger Prince Aaron!
“I won’t live while hiding my power like the Duke did!”
By using any means necessary to develop the land,
and preparing for Emperor Zerone’s invasion that will happen 20 years,
he decides to take back the spot of Emperor which he was robbed of!