From Throne to Classroom: The Warlord Reborn

Leon Thorne wants to make up for the regrets of his first life, when he was Prince Regent of the Solarian Empire and the Emperor betrayed him after he had conquered six nations. He cannot go back to it, because he has been reborn into the modern world as a timid high school student. He drops the cowardice first and puts down Chad Keller, who had been bullying him. In his class is Elise Mercer, who has the face of the fiancée of his past life, and her name as well. She wears a necklace that carries black magic and is doing her harm, and he breaks it and suppresses what is inside. His brother in this life, Jared Thorne, is a gambler with debts, so Leon frightens him into line and then clears the debts. An organisation called the Sanctum, with Dexter Cross in it, goes after the Mercer family's money. Leon brings out the power of a Prime Grandmaster, kills the assassins sent against the Mercers, takes the Sanctum apart, and wins the loyalty of Edward Langdon, the richest man in Riverton.