Reincarnated Devil's Plan For Raising the Strongest Hero

Story:

After being reborn as a low-level demon in hell, Kakyu, a former Japanese, has spent two millennia developing unmatched strength. But out of the blue, he is banished to a "different world," freed from the cruel underworld. While living the lazy life of a dream, Kakyu sees the hamlet he owes everything to being attacked. The village is destroyed even as everyone rushes to the scene. After taking in the lone survivor, Alus, Kakyu decides to nurture him to be the happiest person alive in order to pay back the debt.