Tsuyokute New Saga

A brief description of the manga Tsuyokute New Saga:

After a bloody battle, the swordsman Kyle does kill the Lord of Demons, but he himself is seriously wounded. On the verge of death and life Kyle finds a relic kept in the Lord’s chambers, takes it in his hand and… is transferred to the past.

Having recovered from the initial shock, he decides to use this opportunity to avoid repeating the mistakes of the past and become stronger.