Millimos Saga: Battei Ouji no Tensei Senki summary:
After encountering a train accident, he was reincarnated into another world and ended up as the youngest prince of a weak country in the mountains!?
He was supposed to enjoy a leisurely, slow life, but instead he mastered ``magic'' and ``sacred arts'' on his own and became a military marshal.
Milimos gains two powers that cannot coexist, and gradually becomes embroiled in a national war regardless of his will.