Moto Saikyou No Kenshi Wa, Isekai Mahou Ni Akogareru

Soma, the world's strongest swordsman, is killed in a battle with the toughest opponent imaginable: The Dragon God. On the verge of death, his only wish, to be able to use magic, is granted by the God... But with a catch. He has to be transmigrated to another world, where he must take the form of a six-year-old kid with apparently no skills at all! Thus, the man who was once called the Sword God, the strongest swordsman in history, must secretly harbour all of his former strength while trying to attain the only skill he cares about... Magic.